Add @-mention highlighting, sidebar badge, and push customization (#39)

@username tokens in a sent message are parsed against the room's actual
members (skipping fenced/inline code, so pasted code isn't misread) and
recorded as MessageMention rows, reusing #38's read-tracking and
offline-member broadcast infrastructure rather than building a parallel
notification path:

- Sidebar: a mentioned-and-unread room shows a distinct highlight-
  colored badge instead of (not alongside) the plain unread dot --
  computed the same way as has_unread, just scoped to messages that
  mention the caller, and cleared by the same last_read_at mark-read
  flow.
- Push notifications: a mentioned offline recipient gets "X mentioned
  you: ..." instead of the generic "X: ...", still per-recipient since
  the same message can page some room members and not others.
- Message rendering: a validated @username is highlighted inline,
  implemented by turning it into a `[@username](mention:username)` link
  before markdown parsing and overriding link rendering to style
  `mention:`-scheme links as a span instead of an anchor -- reuses
  markdown-to-jsx's existing parser rather than hand-rolling text-node
  splitting.
- Composer: typing @ opens an autocomplete dropdown of matching room
  members (arrow keys to navigate, Enter/Tab/click to insert, Escape or
  moving the cursor away to dismiss).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/05dbe3775e34_message_mentions_for_at_mention_.py

@@ -0,0 +1,40 @@
+"""message mentions for at-mention highlighting
+
+Revision ID: 05dbe3775e34
+Revises: 8f1b4bf29c5d
+Create Date: 2026-08-17 08:29:40.231511
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '05dbe3775e34'
+down_revision: Union[str, Sequence[str], None] = '8f1b4bf29c5d'
+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('message_mentions',
+    sa.Column('message_id', sa.Uuid(), nullable=False),
+    sa.Column('user_id', sa.Uuid(), nullable=False),
+    sa.ForeignKeyConstraint(['message_id'], ['messages.id'], ),
+    sa.ForeignKeyConstraint(['user_id'], ['users.id'], ),
+    sa.PrimaryKeyConstraint('message_id', 'user_id')
+    )
+    op.create_index(op.f('ix_message_mentions_user_id'), 'message_mentions', ['user_id'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_message_mentions_user_id'), table_name='message_mentions')
+    op.drop_table('message_mentions')
+    # ### end Alembic commands ###