refactor(update)!: make obtainability a property of updates, not a surface

The owner could not tell from the UI what "Buzz Price Tracking" enabled, what the
"Price alert threshold" number meant, or what "Price Alerts" was alerting about.
That was not a copy problem: the implementation exposed our mechanism (a page
scrape) and our SQL predicates as the user's concepts. Two concrete defects came
from the same root:

- the alert population included versions the user already owns (neither the event
  generator nor the query filtered on is_in_library; in the owner's library 28 of
  52 gated versions were already downloaded, so most "alerts" were about files
  already on disk, which cannot become cheaper *for them*);
- a threshold-filtered state list lived in a notification surface, so an empty
  panel had three indistinguishable causes and read as a broken feature.

The information model is now the version plus ownership: cost is shown only where
a decision exists. Owned -> nothing. Not owned and free -> nothing. Not owned and
gated -> the price when it is known, `Paid` without a number when it is not, and
early access keeps its countdown because "free on <date>" decides between waiting
and paying. The numeric threshold has no place in that model: every decision is
categorical (wait / pay / skip), so the setting, the comparison and the whole
alert-state machine are gone.

- both alert-state columns are removed from the schema rather than left dead; a
  database created by an unreleased build has them dropped on open (native
  ALTER TABLE ... DROP COLUMN, guarded), which is a no-op for everyone else
- gate events are emitted only for versions the user does not have, and the
  price-drop event goes with the threshold it belonged to
- both alert endpoints, PriceAlertsHandler and the service-registry adapter field
  it needed are removed: events already reach the UI through the refresh response
- the bell tab, panel, CSS, both entry points, the unread watermark and their
  locales are removed; the setting keeps only the enable flag and the refresh
  interval and is framed as plumbing
- "Price unavailable" is replaced by `Paid`: the gate is certain from the public
  API, only the number is best-effort, and that is our plumbing, not the user's
  problem

Verified against a copy of the owner's real database: 52 gated versions ->
28 owned (now silent) + 24 the feature is actually about; the drop migration ran
and both removed endpoints 404.

static/js/state/index.js

@@ -58,7 +58,6 @@ const DEFAULT_SETTINGS_BASE = Object.freeze({
     hide_early_access_updates: false,
     hide_paid_updates: false,
     price_tracking_enabled: false,
-    price_alert_threshold_buzz: 0,
     price_check_ttl_hours: 24,
     auto_organize_exclusions: [],
     metadata_refresh_skip_paths: [],