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/api.py

@@ -61,6 +61,7 @@ def create_app(
         tracks = db.get_show_tracks(show.id)
         return {
             "show_id": show.id,
+            "episode_number": show.episode_number,
             "week_start": show.week_start.isoformat(),
             "week_end": show.week_end.isoformat(),
             "tracks": tracks,
@@ -80,6 +81,7 @@ def create_app(
         return [
             {
                 "id": s.id,
+                "episode_number": s.episode_number,
                 "week_start": s.week_start.isoformat(),
                 "week_end": s.week_end.isoformat(),
                 "created_at": s.created_at.isoformat(),
@@ -96,6 +98,7 @@ def create_app(
         tracks = db.get_show_tracks(show.id)
         return {
             "show_id": show.id,
+            "episode_number": show.episode_number,
             "week_start": show.week_start.isoformat(),
             "week_end": show.week_end.isoformat(),
             "tracks": tracks,