fix(example-images): read real dimensions for imported videos, fixes #1115

Example videos added through the "Add examples" flow were stored with a
hardcoded 720x1280 entry. The dimension probe next to it only ran for
images (PIL cannot open .mp4/.webm files), so every video entry stayed
portrait regardless of the source. The showcase viewer then sizes its
container straight from that value (--media-aspect in showcase.css), so
landscape clips were letterboxed inside a 9:16 box. CivitAI-sourced
examples were unaffected because their dimensions come from the API.

PIL cannot read video containers, so add a dependency-free reader that
parses the container headers instead: moov/trak/tkhd for ISO base media
(with the sample description as a fallback), Segment/Tracks/Pixel* for
WebM/Matroska, and RIFF/WebP for animated examples saved with a video
extension. The sniffed signature decides which reader runs, so a .mp4
that is really WebM still reports the right size; the extension is only
a fallback. Both readers seek past mdat rather than reading it, so a
large file costs the same as a small one.

Imported entries now record the file's real size and keep the previous
placeholder only when the file cannot be parsed.

Existing libraries keep their wrong entries, so backfill them once via
the existing naming migration: bump CURRENT_NAMING_VERSION to 3 and
repair each model's empty-url entries from the files on disk, then sync
the scanner cache. Only entries with no remote url are touched -- those
have no other source, which makes the rewrite lossless -- and entries
already carrying the right size are left byte-identical, so the pass is
idempotent and a no-op for libraries that never imported a video.

tests/utils/test_video_dimension_probe.py

@@ -0,0 +1,178 @@
+"""Tests for the container-level video dimension probe."""
+
+from __future__ import annotations
+
+import struct
+
+from py.utils.video_metadata import get_video_dimensions
+
+
+def _box(box_type: bytes, payload: bytes) -> bytes:
+    return struct.pack(">I", len(payload) + 8) + box_type + payload
+
+
+def _full_box(box_type: bytes, payload: bytes) -> bytes:
+    """Build a box with a 4-byte version/flags header."""
+
+    return _box(box_type, b"\x00\x00\x00\x00" + payload)
+
+
+def build_mp4(width: int, height: int, *, with_stsd: bool = False) -> bytes:
+    """Build a minimal but structurally valid MP4 holding one video track."""
+
+    mvhd = _full_box(b"mvhd", b"\x00" * 96)
+
+    hdlr = _full_box(b"hdlr", b"\x00" * 4 + b"vide" + b"\x00" * 12)
+
+    tkhd_payload = struct.pack(">IIII", 0, 0, 0, 0) + b"\x00" * 52
+    tkhd_payload += struct.pack(">II", width << 16, height << 16)
+    tkhd = _full_box(b"tkhd", tkhd_payload)
+
+    stbl_children = b""
+    if with_stsd:
+        sample_entry = (
+            b"\x00" * 6 + struct.pack(">H", 1) + struct.pack(">HH", width, height)
+        )
+        stsd = _full_box(b"stsd", struct.pack(">I", 1) + _box(b"avc1", sample_entry))
+        stbl_children = stsd
+
+    minf = _box(b"minf", _box(b"stbl", stbl_children))
+    mdia = _box(b"mdia", hdlr + minf)
+    trak = _box(b"trak", tkhd + mdia)
+    moov = _box(b"moov", mvhd + trak)
+    ftyp = _box(b"ftyp", b"isom" + b"\x00\x00\x02\x00" + b"isomiso2avc1mp41")
+
+    return ftyp + moov
+
+
+def _ebml_vint(value: int) -> bytes:
+    """Encode a value as a minimal-length EBML variable length integer."""
+
+    for length in range(1, 9):
+        if value < (1 << (7 * length)):
+            encoded = value | (1 << (7 * length))
+            return encoded.to_bytes(length, "big")
+    raise ValueError("value too large for an EBML vint")
+
+
+def _ebml_element(element_id: bytes, payload: bytes) -> bytes:
+    return element_id + _ebml_vint(len(payload)) + payload
+
+
+def _uint_element(element_id: int, value: int) -> bytes:
+    length = max(1, (value.bit_length() + 7) // 8)
+    return _ebml_element(
+        element_id.to_bytes(2, "big") if element_id > 0xFF else element_id.to_bytes(1, "big"),
+        value.to_bytes(length, "big"),
+    )
+
+
+def build_webm(width: int, height: int, *, track_type: int = 1) -> bytes:
+    """Build a minimal WebM file holding one TrackEntry."""
+
+    video = _ebml_element(b"\xe0", _uint_element(0xB0, width) + _uint_element(0xBA, height))
+    track_entry = _ebml_element(
+        b"\xae", _uint_element(0x83, track_type) + video
+    )
+    tracks = _ebml_element(b"\x16\x54\xae\x6b", track_entry)
+
+    segment = _ebml_element(b"\x18\x53\x80\x67", tracks)
+    ebml_header = _ebml_element(
+        b"\x1a\x45\xdf\xa3",
+        _uint_element(0x4286, 1) + _ebml_element(b"\x42\x82", b"webm"),
+    )
+
+    return ebml_header + segment
+
+
+def test_mp4_dimensions_come_from_tkhd(tmp_path):
+    video = tmp_path / "landscape.mp4"
+    video.write_bytes(build_mp4(1280, 720))
+
+    assert get_video_dimensions(str(video)) == (1280, 720)
+
+
+def test_mp4_uses_stsd_when_tkhd_is_empty(tmp_path):
+    video = tmp_path / "stsd-only.mp4"
+    video.write_bytes(build_mp4(640, 480, with_stsd=True))
+
+    assert get_video_dimensions(str(video)) == (640, 480)
+
+
+def test_mp4_without_video_track_returns_none(tmp_path):
+    # A moov whose only trak has no mdia box at all.
+    tkhd = _full_box(b"tkhd", b"\x00" * 60)
+    moov = _box(b"moov", _box(b"trak", tkhd))
+    video = tmp_path / "audio-only.mp4"
+    video.write_bytes(moov)
+
+    assert get_video_dimensions(str(video)) is None
+
+
+def test_webm_dimensions(tmp_path):
+    video = tmp_path / "portrait.webm"
+    video.write_bytes(build_webm(720, 1280))
+
+    assert get_video_dimensions(str(video)) == (720, 1280)
+
+
+def test_webm_non_video_track_is_ignored(tmp_path):
+    video = tmp_path / "audio.webm"
+    video.write_bytes(build_webm(720, 1280, track_type=2))
+
+    assert get_video_dimensions(str(video)) is None
+
+
+def test_container_signature_wins_over_extension(tmp_path):
+    """A WebM file named ``.mp4`` is still parsed as WebM."""
+
+    video = tmp_path / "actually-webm.mp4"
+    video.write_bytes(build_webm(480, 832))
+
+    assert get_video_dimensions(str(video)) == (480, 832)
+
+
+def test_webp_renamed_to_mp4_is_read(tmp_path):
+    """Animated WebP examples are frequently saved with a video extension."""
+
+    vp8_payload = b"\x30\x36\x02" + b"\x9d\x01\x2a" + struct.pack("<HH", 450, 800)
+    chunk = b"VP8 " + struct.pack("<I", len(vp8_payload)) + vp8_payload
+    body = b"WEBP" + chunk
+    riff = b"RIFF" + struct.pack("<I", len(body)) + body
+
+    video = tmp_path / "animated.mp4"
+    video.write_bytes(riff)
+
+    assert get_video_dimensions(str(video)) == (450, 800)
+
+
+def test_webp_vp8x_canvas_dimensions(tmp_path):
+    vp8x_payload = b"\x00" * 4 + (449).to_bytes(3, "little") + (799).to_bytes(3, "little")
+    chunk = b"VP8X" + struct.pack("<I", len(vp8x_payload)) + vp8x_payload
+    body = b"WEBP" + chunk
+    riff = b"RIFF" + struct.pack("<I", len(body)) + body
+
+    video = tmp_path / "canvas.mp4"
+    video.write_bytes(riff)
+
+    assert get_video_dimensions(str(video)) == (450, 800)
+
+
+def test_missing_file_returns_none(tmp_path):
+    assert get_video_dimensions(str(tmp_path / "nope.mp4")) is None
+
+
+def test_corrupt_file_returns_none(tmp_path):
+    video = tmp_path / "corrupt.mp4"
+    video.write_bytes(b"\x00\x00\x00\x20ftypisom" + b"\xff" * 64)
+
+    assert get_video_dimensions(str(video)) is None
+
+
+def test_unsupported_extension_without_video_signature_returns_none(tmp_path):
+    """A non-video file is not probed just because of a video-like name."""
+
+    video = tmp_path / "clip.avi"
+    video.write_bytes(b"RIFF\x00\x00\x00\x00AVI LIST\x00\x00\x00\x00")
+
+    assert get_video_dimensions(str(video)) is None