Fix the real cause of out-of-order messages: now() vs clock_timestamp() (#45)

Root cause, confirmed live against Postgres and reproduced end-to-end
through two real WebSocket connections: ws/chat.py shares one AsyncSession
for a whole connection's lifetime. A read-only action (e.g. a "join"
frame's membership check) can leave a transaction open with nothing to
commit it until the next write. Postgres's now()/CURRENT_TIMESTAMP
returns that transaction's *start* time in that case, not the actual
statement's -- so a reply sent after any idle/reading period got
timestamped to when the idle period started, sorting it before messages
that were genuinely sent earlier. This is independent of the two earlier
#45 fixes (missing ORDER BY tiebreakers, a stale-response race on
reload) -- both were real bugs, but this was the actual mechanism behind
"my message appears before theirs even though theirs was sent first."

Switched Message.created_at and MessageReaction.created_at from
func.now() to func.clock_timestamp(), which always reflects the actual
moment of execution regardless of how long the transaction has been
open. Migration is a plain column-default change -- no table rewrite, no
lock risk, round-trips cleanly.

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

backend/alembic/versions/ffdd409227bb_message_and_reaction_timestamps_use_.py

@@ -0,0 +1,37 @@
+"""message and reaction timestamps use clock_timestamp not now
+
+Revision ID: ffdd409227bb
+Revises: 05dbe3775e34
+Create Date: 2026-08-17 13:40:59.440134
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'ffdd409227bb'
+down_revision: Union[str, Sequence[str], None] = '05dbe3775e34'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # Just a catalog default change -- no table rewrite, no lock beyond the
+    # instant one ALTER COLUMN SET DEFAULT always takes. Existing rows are
+    # untouched; only future inserts pick up clock_timestamp().
+    op.alter_column(
+        "messages", "created_at", server_default=sa.text("clock_timestamp()")
+    )
+    op.alter_column(
+        "message_reactions", "created_at", server_default=sa.text("clock_timestamp()")
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.alter_column("messages", "created_at", server_default=sa.text("now()"))
+    op.alter_column("message_reactions", "created_at", server_default=sa.text("now()"))