feat(update): track buzz prices and alert below a threshold

CivitAI's public API deliberately omits prices — paidAccess is trimmed to
{permanent, endsAt} because "pricing belongs to the purchase flow" — but the
public model page embeds the site's own model.getById result, including
paidAccess.terms, in its server-rendered payload. That is read anonymously
(no API key, no internal endpoint, no forged Origin), one request per gated
model, so only the ~2% of models that actually carry a gate pay for it.

- optional capture, off by default: price_tracking_enabled,
  price_alert_threshold_buzz (0 = alert on "became free" only) and
  price_check_ttl_hours; prices refresh on their own TTL and immediately when a
  gate changes, and a failed fetch keeps the stored price instead of blanking it
- versions that stop carrying a gate are marked free (persisted gate_lapsed_at)
  and gate transitions are reported as events on the refresh response, so a
  version already in the library can announce that it became free
- price_alert_state plus a price_drop edge event; new
  GET /api/lm/{type}/updates/price-alerts lists what is under the threshold
- versions tab shows the price (effective, with the list price struck through
  and a Blue Buzz note) and a Free Now badge; an update check toasts the
  transitions in one message
- the parser and the alerts query are unit-tested against a trimmed page
  fixture, and every route definition is now asserted to resolve to a handler

Plan, verification notes and the deviations from it are in
docs/plans/paid-model-price-tracking.md.

tests/utils/test_civitai_page_prices.py

@@ -0,0 +1,91 @@
+"""Tests for the CivitAI model page price parser.
+
+The fixture mirrors a real page payload; the cases that matter are the ones that
+decide whether a user gets a price at all, and whether a lapsed gate is mistaken
+for a live one.
+"""
+
+from pathlib import Path
+
+import pytest
+
+from py.utils.civitai_page_prices import MAX_PAGE_BYTES, parse_model_page_prices
+
+FIXTURE = Path(__file__).parent / "fixtures" / "civitai_model_page_paid.html"
+
+
+@pytest.fixture(scope="module")
+def fixture_html() -> str:
+    return FIXTURE.read_text(encoding="utf-8")
+
+
+def test_parses_prices_for_gated_versions(fixture_html):
+    prices = parse_model_page_prices(fixture_html)
+
+    assert prices is not None
+    # Lapsed gates and never-gated versions must not appear.
+    assert set(prices) == {1001, 1002, 1003}
+
+
+def test_permanent_gate_price(fixture_html):
+    prices = parse_model_page_prices(fixture_html)
+    permanent = prices[1001]
+
+    assert permanent["price_buzz"] == 5000
+    assert permanent["list_price_buzz"] == 5000
+    assert permanent["generation_price_buzz"] == 100
+    assert permanent["accepts_blue_buzz"] is False
+    assert permanent["price_sale_ends_at"] is None
+
+
+def test_timed_gate_uses_sale_price_as_effective(fixture_html):
+    prices = parse_model_page_prices(fixture_html)
+    timed = prices[1002]
+
+    # The stored/list price stays visible for a strikethrough, the effective price
+    # is what a buyer pays now.
+    assert timed["price_buzz"] == 100
+    assert timed["list_price_buzz"] == 125
+    assert timed["accepts_blue_buzz"] is True
+    assert timed["price_sale_ends_at"] == "2999-09-01T00:00:00.000Z"
+    # Generation is bundled with the download tier here (`trialLimit` only).
+    assert timed["generation_price_buzz"] is None
+
+
+def test_timed_gate_without_recorded_end_is_still_priced(fixture_html):
+    prices = parse_model_page_prices(fixture_html)
+
+    assert prices[1003]["price_buzz"] == 300
+    assert prices[1003]["generation_price_buzz"] is None
+
+
+@pytest.mark.parametrize(
+    "html",
+    [
+        None,
+        "",
+        "<html><body>no payload here</body></html>",
+        '<script id="__NEXT_DATA__" type="application/json">{not json</script>',
+        # Valid JSON, but not the shape we need.
+        '<script id="__NEXT_DATA__" type="application/json">{"props":{}}</script>',
+        # Model query present, but no versions.
+        '<script id="__NEXT_DATA__" type="application/json">'
+        '{"props":{"pageProps":{"trpcState":{"json":{"queries":['
+        '{"queryKey":[["model","getById"]],"state":{"data":{}}}]}}}}}</script>',
+    ],
+)
+def test_unusable_payloads_return_none(html):
+    assert parse_model_page_prices(html) is None
+
+
+def test_oversized_page_is_skipped():
+    oversized = "x" * (MAX_PAGE_BYTES + 1)
+    assert parse_model_page_prices(oversized) is None
+
+
+def test_unrelated_first_query_does_not_win(fixture_html):
+    """Query order varies per page; selection is by procedure name."""
+
+    prices = parse_model_page_prices(fixture_html)
+    assert prices is not None
+    assert 1001 in prices