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/routers/custom_emoji.py

@@ -0,0 +1,131 @@
+import uuid
+
+from fastapi import APIRouter, Depends, File, Form, HTTPException, UploadFile
+from fastapi.responses import FileResponse
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+from app.dependencies import get_current_user
+from app.models import User
+from app.schemas.custom_emoji import CustomEmojiRead
+from app.services.custom_emoji_service import (
+    SHORTCODE_PATTERN,
+    CustomEmojiNotFoundError,
+    DuplicateShortcodeError,
+    InvalidShortcodeError,
+    NotEmojiOwnerError,
+    create_custom_emoji,
+    delete_custom_emoji,
+    get_custom_emoji_by_shortcode,
+    list_custom_emoji,
+)
+from app.services.upload_settings_service import format_mb, get_upload_settings
+from app.storage import (
+    ALLOWED_IMAGE_CONTENT_TYPES,
+    UPLOADS_DIR,
+    InvalidImageError,
+    UploadTooLargeError,
+    process_image,
+    read_capped,
+    save_file,
+)
+
+# Small and square -- these render inline in message text/reaction pills at
+# roughly text size, nowhere near message-image or avatar dimensions.
+CUSTOM_EMOJI_MAX_DIMENSION = 128
+
+router = APIRouter(prefix="/api/custom-emoji", tags=["custom-emoji"])
+
+
+@router.post("", response_model=CustomEmojiRead, status_code=201)
+async def upload_custom_emoji_endpoint(
+    shortcode: str = Form(...),
+    file: UploadFile = File(...),
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    shortcode = shortcode.strip().lower()
+    # Checked here, before any file processing/saving, so the common
+    # rejection cases (bad format, name taken) never leave an orphaned
+    # file on disk -- create_custom_emoji below still re-checks both
+    # (the actual source of truth, and the only thing that closes the
+    # TOCTOU race on the uniqueness check).
+    if not SHORTCODE_PATTERN.match(shortcode):
+        raise HTTPException(
+            status_code=400,
+            detail="Shortcode must be 2-30 characters: lowercase letters, numbers, hyphens, underscores",
+        )
+    if await get_custom_emoji_by_shortcode(db, shortcode) is not None:
+        raise HTTPException(status_code=409, detail="An emoji with that shortcode already exists")
+
+    if file.content_type not in ALLOWED_IMAGE_CONTENT_TYPES:
+        raise HTTPException(status_code=400, detail="Unsupported image type")
+
+    upload_settings = await get_upload_settings(db)
+    try:
+        data = await read_capped(file, cap=upload_settings.max_upload_bytes)
+    except UploadTooLargeError:
+        raise HTTPException(
+            status_code=413,
+            detail=f"Image exceeds {format_mb(upload_settings.max_upload_bytes)} limit",
+        )
+
+    try:
+        data, ext = process_image(
+            data, file.content_type, square=True, max_dimension=CUSTOM_EMOJI_MAX_DIMENSION
+        )
+    except InvalidImageError:
+        raise HTTPException(status_code=400, detail="File is not a valid image")
+
+    storage_filename = save_file(data, ext)
+    try:
+        return await create_custom_emoji(
+            db, current_user.id, shortcode, storage_filename, file.content_type
+        )
+    except (InvalidShortcodeError, DuplicateShortcodeError):
+        # Already checked above -- only reachable via the uniqueness
+        # check's TOCTOU race (two uploads of the same new shortcode at
+        # once), not the common case.
+        raise HTTPException(status_code=409, detail="An emoji with that shortcode already exists")
+
+
+@router.get("", response_model=list[CustomEmojiRead])
+async def list_custom_emoji_endpoint(
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    return await list_custom_emoji(db)
+
+
+@router.delete("/{emoji_id}", status_code=204)
+async def delete_custom_emoji_endpoint(
+    emoji_id: uuid.UUID,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    try:
+        await delete_custom_emoji(db, emoji_id, current_user)
+    except CustomEmojiNotFoundError:
+        raise HTTPException(status_code=404, detail="Custom emoji not found")
+    except NotEmojiOwnerError:
+        raise HTTPException(status_code=403, detail="Only the uploader or a site admin can remove this")
+
+
+@router.get("/{shortcode}/image")
+async def get_custom_emoji_image_endpoint(
+    shortcode: str,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    emoji = await get_custom_emoji_by_shortcode(db, shortcode)
+    if emoji is None:
+        raise HTTPException(status_code=404, detail="Custom emoji not found")
+    return FileResponse(
+        UPLOADS_DIR / emoji.storage_filename,
+        media_type=emoji.content_type,
+        # Site-wide and rarely changed, but a shortcode can be deleted and
+        # re-uploaded with different image data -- short-cache like the
+        # avatar endpoint, not `immutable` like content-addressed message
+        # images.
+        headers={"Cache-Control": "private, max-age=300"},
+    )