chore(db): renumber the runtime migration to 0026

The concurrent subtitles milestone carries 0024_subtitles.sql and
0025_subtitle_settings.sql. Two migrations claiming version 24 do not
conflict in git — the filenames differ — so both would land on main and
sqlx would see a duplicate version. Renumbering here is the half that
does not depend on the other milestone acting.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/arr-db/migrations/0026_series_runtime.sql

@@ -0,0 +1,6 @@
+-- §5.5 as amended by #187/#208: size bands are rates against a 45-minute
+-- reference runtime, scaled by the series' minutes per episode. NULL is a
+-- missing runtime — TMDB's episode_run_time is frequently empty — and means
+-- the bands apply unscaled.
+ALTER TABLE series ADD COLUMN runtime_minutes INTEGER
+    CHECK (runtime_minutes IS NULL OR runtime_minutes > 0);