fix(update): decouple price capture from the metadata TTL

Found in a real instance: after enabling price tracking, a normal "Check updates"
captured exactly one price out of 718 models, so the alerts panel looked broken
while the log said the refresh completed.

Price capture only ran when the version list was re-fetched, so it inherited the
metadata TTL: with 24 h metadata and 24 h price TTLs, only the handful of models
whose metadata happened to be stale that round were ever priced.

- the cached record already carries the gate, so the price pass now runs off
  whichever version list is available (freshly fetched or stored) and applies the
  result without touching last_checked_at, so a price-only pass cannot silently
  extend the metadata TTL
- a failed attempt now satisfies the price TTL, so a mature model whose page no
  host will serve is not retried on every single update check
- an explicitly forced check re-prices within the TTL

Verified by copying a real instance's update DB into a sandbox and running a
non-forced check: bulk metadata fetches 0 (version lists entirely from cache)
while priced versions went 1 -> 20 and the panel listed 19 alerts.

docs/plans/paid-model-price-tracking.md

@@ -652,3 +652,22 @@ Recorded options for mature models, deliberately **not** implemented:
 "convenience" — the page route is demonstrably unreliable (one host challenges non-browser clients,
 the other two hide mature models from anonymous visitors), so a supported field is the only way for
 any third-party tool to show prices for the models where creators monetize most.
+
+### 11.11 Price capture is independent of the metadata TTL
+
+Found in a real instance: after enabling price tracking, a normal "Check updates" captured exactly
+**one** price out of 718 models. The price capture only ran when the *version list* was re-fetched
+(`refresh_succeeded`), so it inherited the metadata TTL: with 24 h metadata and 24 h price TTLs, only
+the 6 models whose metadata happened to be stale were ever priced.
+
+The cached record already carries the gate, so the price pass now runs off whichever version list is
+available — freshly fetched or stored — and applies the result without touching `last_checked_at`, so
+a price-only pass cannot silently extend the metadata TTL. Two related semantics:
+
+* A **failed** attempt (`price_check_attempted_at`) satisfies the price TTL, so a mature model whose
+  page no host will serve is not retried on every single update check.
+* An explicitly **forced** check re-prices within the TTL (`_should_fetch_prices(..., force=True)`).
+
+Verified by copying a real instance's update DB into a sandbox and running a non-forced check:
+`bulk metadata fetches: 0` (version lists came entirely from cache) while priced versions went
+**1 → 20** and the panel listed 19 alerts.