Add custom emoji support (#18)

Site-wide, any user can upload -- usable both as reactions and inline
in message text via :shortcode:, alongside the existing built-in
Unicode picker. A :shortcode: reference is stored/sent as literal
text (same as the built-in shortcode convention) and resolved to an
image at render time, so it degrades to plain text if the emoji is
later deleted.

Backend: new custom_emoji table (shortcode unique, sized to fit
MessageReaction.emoji's existing column alongside its colons), upload/
list/delete endpoints (delete restricted to uploader or site admin).

Frontend: a CustomEmojiProvider context feeds a new "Custom" category
in the emoji picker (inline upload + hover-to-remove), extends the
composer's shortcode autocomplete, and a shared EmojiGlyph resolver
renders custom emoji wherever a value can appear -- message text,
reaction pills, and the picker itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/ws/chat.py

@@ -311,7 +311,11 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
                         envelope.room_id is None
                         or envelope.message_id is None
                         or not envelope.emoji
-                        or len(envelope.emoji) > 8
+                        # #18: a raw unicode glyph never gets close to this,
+                        # but a custom emoji reaction is stored as its
+                        # literal `:shortcode:` text (see MessageReaction.emoji's
+                        # String(32) column, which this matches exactly).
+                        or len(envelope.emoji) > 32
                     ):
                         await websocket.send_json(
                             {"type": "error", "detail": "room_id, message_id, and emoji required"}