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/models/__init__.py

@@ -1,6 +1,7 @@
 from app.models.admin_audit_log import AdminAuditLog
 from app.models.api_token import ApiToken
 from app.models.base import Base
+from app.models.custom_emoji import CustomEmoji
 from app.models.custom_theme import CustomTheme
 from app.models.event_subscription import EventSubscription
 from app.models.invite import InviteStatus
@@ -46,5 +47,6 @@ __all__ = [
     "WebhookIncoming",
     "EventSubscription",
     "CustomTheme",
+    "CustomEmoji",
     "LinkPreview",
 ]