feat: scale size bands by episode runtime

Implements #209 per §5.5 as amended by #208: a band's floor and target
are rates against a 45-minute reference runtime, scaled by the series'
minutes per episode. A missing or zero runtime applies the bands
unscaled, and movies are never scaled. The runtime is stored on the
series row (new migration), filled on add and by the metadata refresh,
which never blanks a known value against TMDB's frequently-empty
episode_run_time. Composes with #210: allow_below_floor waives against
the scaled floor.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/arr-db/migrations/0024_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);