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>

frontend/src/components/MessageContent.css

@@ -0,0 +1,13 @@
+/* #18: em-relative, deliberately -- renders correctly inline in message
+   text, inside a reaction pill, and inside the emoji picker's grid without
+   a separate override per context, since each of those already sets its
+   own font-size and this just tracks it. Kept in this file (imported
+   directly by MessageContent.tsx) rather than MessageList.css so it's
+   loaded wherever MessageContent renders -- FilePreviewModal and HelpPage
+   included, not just the message list. */
+.message-custom-emoji {
+  height: 1.2em;
+  width: 1.2em;
+  object-fit: contain;
+  vertical-align: -0.25em;
+}