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>

.sqlx/query-fffd6c2609b3389aa94d41cdde6e0e9113ce6c60498772b1fc056ffa037d1166.json

@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT s.runtime_minutes FROM series s\n           WHERE s.id = (SELECT s2.series_id FROM episodes e\n                         JOIN seasons s2 ON s2.id = e.season_id\n                         WHERE e.id = ?)",
+  "describe": {
+    "columns": [
+      {
+        "name": "runtime_minutes",
+        "ordinal": 0,
+        "type_info": "Integer",
+        "origin": {
+          "Table": {
+            "table": "series",
+            "name": "runtime_minutes"
+          }
+        }
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      true
+    ]
+  },
+  "hash": "fffd6c2609b3389aa94d41cdde6e0e9113ce6c60498772b1fc056ffa037d1166"
+}