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_poller.py

@@ -93,6 +93,89 @@ async def test_poll_once_removes_unliked_tracks():
     assert call_args[0][1] == [1]
 
 
+@pytest.mark.asyncio
+async def test_poll_once_auto_assigns_episode_number():
+    mock_sc = AsyncMock()
+    mock_sc.resolve_user.return_value = 206979918
+    mock_sc.fetch_likes.return_value = [
+        _make_track(1, "2026-03-14T01:00:00+00:00"),
+    ]
+
+    mock_db = MagicMock()
+    mock_show = MagicMock()
+    mock_show.id = 5
+    mock_show.episode_number = None
+    mock_db.get_or_create_show.return_value = mock_show
+    mock_db.get_latest_episode_number.return_value = 530
+
+    poller = Poller(
+        db=mock_db,
+        soundcloud=mock_sc,
+        soundcloud_user="nicktherat",
+        show_day=2,
+        show_hour=22,
+        poll_interval=3600,
+    )
+
+    await poller.poll_once()
+
+    mock_db.update_show_episode_number.assert_called_once_with(5, 531)
+
+
+@pytest.mark.asyncio
+async def test_poll_once_skips_numbering_when_no_history():
+    mock_sc = AsyncMock()
+    mock_sc.resolve_user.return_value = 206979918
+    mock_sc.fetch_likes.return_value = []
+
+    mock_db = MagicMock()
+    mock_show = MagicMock()
+    mock_show.id = 1
+    mock_show.episode_number = None
+    mock_db.get_or_create_show.return_value = mock_show
+    mock_db.get_latest_episode_number.return_value = None
+
+    poller = Poller(
+        db=mock_db,
+        soundcloud=mock_sc,
+        soundcloud_user="nicktherat",
+        show_day=2,
+        show_hour=22,
+        poll_interval=3600,
+    )
+
+    await poller.poll_once()
+
+    mock_db.update_show_episode_number.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_poll_once_skips_numbering_when_already_assigned():
+    mock_sc = AsyncMock()
+    mock_sc.resolve_user.return_value = 206979918
+    mock_sc.fetch_likes.return_value = []
+
+    mock_db = MagicMock()
+    mock_show = MagicMock()
+    mock_show.id = 1
+    mock_show.episode_number = 530
+    mock_db.get_or_create_show.return_value = mock_show
+
+    poller = Poller(
+        db=mock_db,
+        soundcloud=mock_sc,
+        soundcloud_user="nicktherat",
+        show_day=2,
+        show_hour=22,
+        poll_interval=3600,
+    )
+
+    await poller.poll_once()
+
+    mock_db.get_latest_episode_number.assert_not_called()
+    mock_db.update_show_episode_number.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_poll_once_full_refresh():
     mock_sc = AsyncMock()