feat: the artist picker on Patreon and SubscribeStar too — and the Patreon name is canon (milestone 429)

Operator: "yes add the artist picker to patreon and subscribestar too. but
generally we treat the patreon name as the canon"

- Every add now goes through the panel. On Patreon/SubscribeStar it opens
  on the creator's display name (read only when the panel opens:
  probe?names=1), searches FC's artists with it, and auto-picks a match.
  An untouched URL handle sends no name, so the server still resolves it.
- Patreon is canon: joining a Patreon source to an artist known by another
  name offers "Rename “x” to the Patreon name “X”", ticked by default.
  quick-add's use_platform_name renames server-side from the name it reads
  itself; name only, the slug never moves (#130); never to a URL handle when
  the name can't be read; ignored on SubscribeStar and Discord.
- _platform_display_name returns None rather than the handle, bounded by the
  same 6s lookup budget as Discord's names.
- panelDefaults / addRequest / renameOffer replace the Discord-only helpers.

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

backend/app/api/extension.py

@@ -95,7 +95,9 @@ async def probe_source():
             return _bad("unauthorized", status=401)
         # crypto lets a Discord probe name the server and channel with the
         # stored token; every other platform ignores it.
-        result = await ExtensionService(session, _get_crypto()).probe(url)
+        result = await ExtensionService(session, _get_crypto()).probe(
+            url, names=request.args.get("names") in ("1", "true"),
+        )
     return jsonify(result)
 
 
@@ -116,6 +118,9 @@ async def quick_add_source():
     artist_name = body.get("artist_name")
     if artist_name is not None and not isinstance(artist_name, str):
         return _bad("invalid_body", detail="artist_name must be a string")
+    # Patreon is canon: adding a Patreon source to an existing artist can take
+    # the creator's Patreon display name (name only; the slug never moves).
+    use_platform_name = body.get("use_platform_name") is True
 
     from .credentials import _get_crypto
 
@@ -127,6 +132,7 @@ async def quick_add_source():
             # stored credential (else it falls back to the URL handle). #130.
             result = await ExtensionService(session, _get_crypto()).quick_add_source(
                 url, artist_id=artist_id, artist_name=artist_name,
+                use_platform_name=use_platform_name,
             )
         except UnknownArtistError as exc:
             return _bad("not_found", detail=str(exc), status=404)