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/custom_emoji.py

@@ -0,0 +1,28 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, String, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class CustomEmoji(Base):
+    __tablename__ = "custom_emoji"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    # #18: site-wide, not room-scoped -- kept globally unique so a bare
+    # `:shortcode:` in any message/reaction is unambiguous without also
+    # knowing which room it was posted in. 30 chars, not 32 -- the stored
+    # *reference* in MessageReaction.emoji (String(32)) is the shortcode
+    # wrapped in colons, so this is sized to leave room for both without
+    # widening that column.
+    shortcode: Mapped[str] = mapped_column(String(30), unique=True, index=True, nullable=False)
+    storage_filename: Mapped[str] = mapped_column(String(64), nullable=False)
+    content_type: Mapped[str] = mapped_column(String(50), nullable=False)
+    uploaded_by: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    uploader = relationship("User")