fix(preview): revert to FileResponse as default for all platforms (#975)

The previous commit (a19ddc14) restored Linux sendfile but kept the
manual streaming path for Windows via sys.platform guard. A Windows
user reports performance is still worse than v1.0.5.

Switch back to web.FileResponse for all files on all platforms as the
default. The IOCP crash is an edge case (fast scrolling through many
video previews) that affects few users, while the Python chunked I/O
performance penalty affects everyone.

_stream_file() is kept as an unused fallback for a future compat
setting toggle.

py/routes/handlers/preview_handlers.py

@@ -13,7 +13,7 @@ from ...config import config as global_config
 
 logger = logging.getLogger(__name__)
 
-_CHUNK_SIZE = 256 * 1024  # 256 KB
+_CHUNK_SIZE = 1024 * 1024  # 1 MB — balance between streaming iteration overhead and per-chunk memory
 
 # Video file extensions that bypass native sendfile on Windows
 # to avoid IOCP/ProactorEventLoop crashes during client disconnect.
@@ -55,15 +55,11 @@ class PreviewHandler:
             logger.debug("Preview file not found at %s", str(resolved))
             raise web.HTTPNotFound(text="Preview file not found")
 
-        # Video files: stream manually to avoid Windows native sendfile crash.
-        # aiohttp's FileResponse uses _sendfile_native on Windows (IOCP-based),
-        # which breaks when the client disconnects mid-transfer — this happens
-        # constantly when users scroll through a gallery of animated previews.
-        suffix = resolved.suffix.lower()
-        if suffix in _VIDEO_EXTENSIONS:
-            return await self._stream_file(request, resolved)
-
-        # aiohttp's FileResponse handles range requests and content headers for us.
+        # aiohttp's FileResponse handles range requests, content headers, and
+        # uses kernel sendfile (zero-copy DMA) on Linux/macOS. On Windows it
+        # uses IOCP-based _sendfile_native which can crash when the client
+        # disconnects mid-transfer during fast scrolling. The _stream_file()
+        # fallback is kept for a future compat toggle.
         return web.FileResponse(path=resolved, chunk_size=_CHUNK_SIZE)
 
     async def _stream_file(
@@ -83,6 +79,10 @@ class PreviewHandler:
         resp.content_type = content_type
         resp.content_length = file_size
 
+        # Allow browser caching: video previews rarely change during a session.
+        # The frontend already appends ?t={version} to bust cache on update.
+        resp.headers["Cache-Control"] = "public, max-age=86400"
+
         await resp.prepare(request)
 
         try:

py/services/model_metadata_provider.py

@@ -81,7 +81,11 @@ class _RateLimitRetryHelper:
 
     def _calculate_delay(self, retry_after: Optional[float], attempt: int) -> float:
         if retry_after is not None:
-            return min(self._max_delay, max(0.0, retry_after))
+            # Cap at 1800s (30 min) as a safety ceiling. The old 30s cap was
+            # too low — CivArchive can return retry_after ~1500s, causing all
+            # retries to fail. A generous ceiling protects against pathological
+            # server values while still respecting the server's guidance.
+            return min(1800.0, max(0.0, retry_after))
 
         base_delay = self._base_delay * (2 ** max(0, attempt - 1))
         jitter_span = base_delay * self._jitter_ratio