feat(db): flag episodes that vanish upstream

crates/arr-db/migrations/0019_vanished_episodes.sql

@@ -0,0 +1,7 @@
+-- #122. TMDB deletes and renumbers episodes. An episode that vanished
+-- upstream and has no file is deleted outright by the refresh; one that has
+-- a file is never deleted — `media_files.path` is UNIQUE and its owner is
+-- polymorphic, so dropping the row would orphan a real file — and instead
+-- carries this flag as the conflict the operator resolves.
+ALTER TABLE episodes ADD COLUMN vanished INTEGER NOT NULL DEFAULT 0
+    CHECK (vanished IN (0, 1));