feat: add historical backfill with --init CLI and episode numbering

Adds a --init mode that seeds the database with past shows from a given
anchor episode/date forward, batch-fetching likes from SoundCloud and
partitioning them into weekly buckets. Episode numbers are tracked in
the shows table and auto-incremented by the poller for new shows.

Includes full API documentation (docs/api.md) and updated README.

Made-with: Cursor

src/ntr_fetcher/models.py

@@ -21,6 +21,7 @@ class Show:
     week_start: datetime
     week_end: datetime
     created_at: datetime
+    episode_number: int | None = None
 
 
 @dataclass(frozen=True)