Fix Jellyfin parent browsing and release dedupe

- List seasons or episodes correctly for Jellyfin parent item queries
- Deduplicate episode availability by logical episode identity
- Improve audiobook title fallbacks and existing item updates

migrations/sql/20260615230358_dedupe_episode_availability_logically.sql

@@ -0,0 +1,36 @@
+-- +goose Up
+-- episode_availability records "this logical episode has already been
+-- available in this library." The original primary key only covered the
+-- catalog episode_id, so a series re-ID or episode-row re-mint (e.g. a local
+-- series gaining a provider match) could remap that id and make an
+-- already-present episode look newly available. Collapse any existing
+-- duplicates and enforce the logical identity going forward.
+WITH ranked AS (
+    SELECT
+        library_id,
+        episode_id,
+        row_number() OVER (
+            PARTITION BY library_id, series_id, episode_key
+            ORDER BY available_at ASC, created_at ASC, episode_id ASC
+        ) AS rn
+    FROM public.episode_availability
+)
+DELETE FROM public.episode_availability ea
+USING ranked r
+WHERE ea.library_id = r.library_id
+  AND ea.episode_id = r.episode_id
+  AND r.rn > 1;
+
+CREATE UNIQUE INDEX episode_availability_logical_episode_key
+    ON public.episode_availability (library_id, series_id, episode_key);
+
+-- The non-unique (library_id, series_id, episode_key DESC) index is now
+-- redundant: the unique index above covers the same leading columns (Postgres
+-- scans it backwards for DESC reads) and the table is insert-only, so nothing
+-- reads it. Drop it rather than carry two indexes over the same tuple.
+DROP INDEX IF EXISTS public.episode_availability_series_idx;
+
+-- +goose Down
+CREATE INDEX episode_availability_series_idx
+    ON public.episode_availability (library_id, series_id, episode_key DESC);
+DROP INDEX IF EXISTS public.episode_availability_logical_episode_key;