Merge #222: one sidecar per language, enforced in the schema

Closes #222

crates/arr-db/migrations/0029_one_sidecar_per_language.sql

@@ -0,0 +1,41 @@
+-- #222. DESIGN.md §15, as amended: a language is satisfied by exactly one
+-- sidecar. No filename segment distinguishes forced from plain from SDH, so
+-- two sidecar rows for one (media file, language) describe two files
+-- competing for one name. The schema says so now rather than leaving it to
+-- the API's path check.
+--
+-- Embedded rows are deliberately untouched. They describe tracks inside the
+-- video, not files on disk, and several can legitimately coexist for one
+-- language — a plain track and a forced one, say. Their key stays
+-- `(media_file_id, language, forced, sdh)`.
+
+-- A database that predates the constraint may already hold a duplicate from
+-- a manual grab that beat the path check: a fetched `.pt-PT.srt` beside a
+-- translated `.pt-PT.mt.srt`, for instance. Resolve rather than fail. Of the
+-- rows for one (media file, language): keep a real subtitle over a machine
+-- translation, and of two of the same kind the newest.
+--
+-- The files stay on disk. Deleting a viewer's subtitle during a migration is
+-- worse than leaving an orphan, and the manual delete (#218, #223) cleans one
+-- up on request.
+DELETE FROM subtitle_files
+WHERE  path IS NOT NULL
+  AND  id NOT IN (
+           SELECT id
+           FROM   (
+               SELECT id,
+                      ROW_NUMBER() OVER (
+                          PARTITION BY media_file_id, language
+                          ORDER BY (origin = 'translated') ASC,
+                                   created_at DESC,
+                                   id DESC
+                      ) AS place
+               FROM   subtitle_files
+               WHERE  path IS NOT NULL
+           )
+           WHERE place = 1
+       );
+
+CREATE UNIQUE INDEX subtitle_files_one_sidecar
+    ON subtitle_files (media_file_id, language)
+    WHERE path IS NOT NULL;