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/alembic/versions/a318850726ee_add_custom_emoji.py

@@ -0,0 +1,43 @@
+"""add custom emoji
+
+Revision ID: a318850726ee
+Revises: 319c30e24cd9
+Create Date: 2026-08-28 20:34:27.293658
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a318850726ee'
+down_revision: Union[str, Sequence[str], None] = '319c30e24cd9'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('custom_emoji',
+    sa.Column('id', sa.Uuid(), nullable=False),
+    sa.Column('shortcode', sa.String(length=30), nullable=False),
+    sa.Column('storage_filename', sa.String(length=64), nullable=False),
+    sa.Column('content_type', sa.String(length=50), nullable=False),
+    sa.Column('uploaded_by', sa.Uuid(), nullable=False),
+    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('now()'), nullable=False),
+    sa.ForeignKeyConstraint(['uploaded_by'], ['users.id'], ),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_custom_emoji_shortcode'), 'custom_emoji', ['shortcode'], unique=True)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_custom_emoji_shortcode'), table_name='custom_emoji')
+    op.drop_table('custom_emoji')
+    # ### end Alembic commands ###