Targeted-search backoff and release-date gating (#90)

crates/arr-db/migrations/0008_movie_digital_release.sql

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+-- TMDB's earliest digital release date is the gate for targeted searches
+-- (§6.2). Keep it with title metadata so a newly announced date resets the
+-- search backoff.
+ALTER TABLE movies ADD COLUMN digital_release TEXT;

crates/arr-db/migrations/0009_movie_metadata_refresh.sql

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+-- Bounds how often targeted search pays for a TMDB call per movie,
+-- independent of the search backoff schedule (§6.2): a title on a
+-- day-long backoff, or one with no digital release date yet, must not
+-- cost a TMDB call every 30 s tick.
+ALTER TABLE movies ADD COLUMN metadata_refreshed_at TEXT;