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

tests/test_db.py

@@ -226,6 +226,52 @@ def test_add_track_to_show_at_position(db):
     assert tracks[2]["track_id"] == 2
 
 
+def test_get_or_create_show_with_episode_number(db):
+    week_start = datetime(2026, 1, 8, 3, 0, 0, tzinfo=timezone.utc)
+    week_end = datetime(2026, 1, 15, 3, 0, 0, tzinfo=timezone.utc)
+    show = db.get_or_create_show(week_start, week_end, episode_number=521)
+    assert show.episode_number == 521
+    show2 = db.get_or_create_show(week_start, week_end)
+    assert show2.id == show.id
+    assert show2.episode_number == 521
+
+
+def test_get_or_create_show_updates_episode_number(db):
+    week_start = datetime(2026, 1, 8, 3, 0, 0, tzinfo=timezone.utc)
+    week_end = datetime(2026, 1, 15, 3, 0, 0, tzinfo=timezone.utc)
+    show = db.get_or_create_show(week_start, week_end)
+    assert show.episode_number is None
+    show2 = db.get_or_create_show(week_start, week_end, episode_number=521)
+    assert show2.id == show.id
+    assert show2.episode_number == 521
+
+
+def test_get_latest_episode_number(db):
+    assert db.get_latest_episode_number() is None
+    db.get_or_create_show(
+        datetime(2026, 1, 8, 3, 0, 0, tzinfo=timezone.utc),
+        datetime(2026, 1, 15, 3, 0, 0, tzinfo=timezone.utc),
+        episode_number=521,
+    )
+    assert db.get_latest_episode_number() == 521
+    db.get_or_create_show(
+        datetime(2026, 1, 15, 3, 0, 0, tzinfo=timezone.utc),
+        datetime(2026, 1, 22, 3, 0, 0, tzinfo=timezone.utc),
+        episode_number=522,
+    )
+    assert db.get_latest_episode_number() == 522
+
+
+def test_update_show_episode_number(db):
+    week_start = datetime(2026, 1, 8, 3, 0, 0, tzinfo=timezone.utc)
+    week_end = datetime(2026, 1, 15, 3, 0, 0, tzinfo=timezone.utc)
+    show = db.get_or_create_show(week_start, week_end)
+    assert show.episode_number is None
+    db.update_show_episode_number(show.id, 521)
+    show2 = db.get_or_create_show(week_start, week_end)
+    assert show2.episode_number == 521
+
+
 def test_has_track_in_show(db):
     week_start = datetime(2026, 3, 13, 2, 0, 0, tzinfo=timezone.utc)
     week_end = datetime(2026, 3, 20, 2, 0, 0, tzinfo=timezone.utc)