feat: the Discord Add panel's artist field autocompletes against FabledCurator's artists (milestone 429)

Operator, after the first live run: "I need the extension to offer an
autofill search function so it's easier to match an entry with an existing
artist."

- The field searches as soon as the panel opens (the server name, usually)
  and on every keystroke; matches list under it, with ↑/↓, Enter/Tab to pick,
  Esc to close, and a last "+ New artist" row.
- Inline autofill: the rest of the top match is filled in and selected, so
  typing on replaces it and Tab/Enter accepts it. Backspacing never refills.
- A result whose name IS the text, spacing and case aside, is picked on its
  own; the hint says in green which existing artist the source will join.
- /api/artists/autocomplete also matches ignoring spacing and punctuation
  ("Tamada Heijun" finds "TamadaHeijun"), ranked just below an exact match.
  The web UI's artist search gets it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_artist_service_find_or_create.py

@@ -50,6 +50,34 @@ async def test_autocomplete_ranks_exact_prefix_substring(db):
     assert "Bob" not in names
 
 
+@pytest.mark.asyncio
+async def test_autocomplete_ignores_spacing_and_punctuation(db):
+    """The same creator is spelled differently per platform — a Discord server
+    "Tamada Heijun" must find the Patreon artist "TamadaHeijun" (milestone 429)."""
+    db.add_all([
+        Artist(name="TamadaHeijun", slug="tamadaheijun"),
+        Artist(name="Sabu Art", slug="sabu-art"),
+        Artist(name="Bob", slug="bob"),
+    ])
+    await db.flush()
+    svc = ArtistService(db)
+    assert [r.name for r in await svc.autocomplete("Tamada Heijun")] == ["TamadaHeijun"]
+    assert [r.name for r in await svc.autocomplete("sabu_art")] == ["Sabu Art"]
+    # All punctuation: no squashed arm, so it does not match everyone.
+    assert await svc.autocomplete("--") == []
+
+
+@pytest.mark.asyncio
+async def test_autocomplete_ranks_an_exact_match_ignoring_spacing_above_a_prefix(db):
+    db.add_all([
+        Artist(name="Sabu Artworks", slug="sabu-artworks"),
+        Artist(name="SabuArt", slug="sabuart"),
+    ])
+    await db.flush()
+    names = [r.name for r in await ArtistService(db).autocomplete("sabu art")]
+    assert names == ["SabuArt", "Sabu Artworks"]
+
+
 @pytest.mark.asyncio
 async def test_autocomplete_empty_query_returns_empty(db):
     svc = ArtistService(db)