feat(arr): add subtitle tables

DESIGN.md §15 needs two shapes: what subtitles exist for a media file,
and what arr has tried per wanted language. An embedded track carries no
path — it is recorded because it satisfies a language, not because there
is a file — and the CHECK constraints tie provider, engine and path to
the origin so an impossible row cannot be written.

crates/arr-db/migrations/0024_subtitles.sql

@@ -0,0 +1,81 @@
+-- #186. What subtitles exist for a media file, and what arr has tried per
+-- wanted language (DESIGN.md §15). Settings rows (#198) and the budget
+-- buckets (#197) are deliberately absent.
+
+-- One row per subtitle arr knows about. An embedded track that was never
+-- extracted has no `path`: it is recorded because it satisfies a language,
+-- not because there is a file. Everything else is a sidecar next to the
+-- video (§15, §7.4), so the path is the row's identity on disk.
+CREATE TABLE subtitle_files (
+    id            INTEGER PRIMARY KEY,
+    media_file_id INTEGER NOT NULL REFERENCES media_files (id) ON DELETE CASCADE,
+    -- As `arr_core::Language` spells it: `pt-PT`, `pt-BR`, `en`.
+    language      TEXT NOT NULL,
+    origin        TEXT NOT NULL
+                  CHECK (origin IN ('embedded', 'extracted', 'provider', 'translated')),
+    -- §15. The named provider a fetch came from, and the candidate id it was
+    -- ranked as, so a re-fetch of the same candidate is recognisable.
+    provider      TEXT,
+    candidate_id  TEXT,
+    -- §15. The translation backend that produced it, when machine made.
+    engine        TEXT,
+    -- §15. A forced track never satisfies a want; an SDH one does, ranked
+    -- below a plain subtitle. Both are facts about the file, so both are
+    -- stored and the ranking rule stays in `arr-core`.
+    forced        INTEGER NOT NULL DEFAULT 0 CHECK (forced IN (0, 1)),
+    sdh           INTEGER NOT NULL DEFAULT 0 CHECK (sdh IN (0, 1)),
+    -- §15. `alass` reports no confidence, so its output is accepted unless
+    -- implausible. `synced` is an accepted shift; `sync_rejected` means the
+    -- unsynced original was kept and this file is flagged. Never both.
+    synced        INTEGER NOT NULL DEFAULT 0 CHECK (synced IN (0, 1)),
+    sync_rejected INTEGER NOT NULL DEFAULT 0 CHECK (sync_rejected IN (0, 1)),
+    path          TEXT UNIQUE,
+    created_at    TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    updated_at    TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    CHECK ((path IS NULL) = (origin = 'embedded')),
+    CHECK ((provider IS NOT NULL) = (origin = 'provider')),
+    CHECK (candidate_id IS NULL OR provider IS NOT NULL),
+    CHECK ((engine IS NOT NULL) = (origin = 'translated')),
+    CHECK (NOT (synced = 1 AND sync_rejected = 1))
+) STRICT;
+
+-- §15's satisfaction check is "is there a subtitle in this language", asked
+-- per file per tick.
+CREATE INDEX subtitle_files_media_file ON subtitle_files (media_file_id, language);
+
+-- An embedded track is identified by what it satisfies, not by a file, so a
+-- second probe of the same video re-records it rather than duplicating it.
+CREATE UNIQUE INDEX subtitle_files_embedded
+    ON subtitle_files (media_file_id, language, forced, sdh)
+    WHERE origin = 'embedded';
+
+-- One row per (media file, wanted language). What the reconcile loop reads to
+-- decide whether a language is a gap, what backs a failing provider off, and
+-- what the missing-subtitles queue renders.
+--
+-- `capped` is a state and not an error (§15): being at a daily allowance is
+-- something the queue shows. The allowance itself is #197 and not here.
+-- `unavailable` is the end of the line — no provider has it and there is no
+-- text source to translate from, so retrying costs budget for nothing.
+CREATE TABLE subtitle_attempts (
+    media_file_id   INTEGER NOT NULL REFERENCES media_files (id) ON DELETE CASCADE,
+    language        TEXT NOT NULL,
+    state           TEXT NOT NULL DEFAULT 'wanted'
+                    CHECK (state IN ('wanted', 'satisfied', 'failed', 'capped', 'unavailable')),
+    attempts        INTEGER NOT NULL DEFAULT 0 CHECK (attempts >= 0),
+    last_attempt_at TEXT,
+    -- Why the last attempt failed, for the queue and for backoff. Cleared
+    -- when an attempt succeeds.
+    last_failure    TEXT,
+    created_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    updated_at      TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    PRIMARY KEY (media_file_id, language),
+    CHECK ((attempts = 0) = (last_attempt_at IS NULL))
+) STRICT;
+
+-- §8, §15. The work list is every language that is not satisfied yet, least
+-- recently attempted first within it; the ordering across files is
+-- newest-import-first and comes from the join to `media_files`.
+CREATE INDEX subtitle_attempts_pending
+    ON subtitle_attempts (state, last_attempt_at)
+    WHERE state <> 'satisfied';