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/app/models/message_reaction.py

@@ -20,8 +20,11 @@ class MessageReaction(Base):
     message_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("messages.id"), index=True, nullable=False)
     user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
     emoji: Mapped[str] = mapped_column(String(32), nullable=False)
+    # clock_timestamp(), not now() -- same transaction-pinning hazard as
+    # Message.created_at (see that column's comment); toggle_reaction runs
+    # through the same long-lived, shared WS session.
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), server_default=func.now(), nullable=False
+        DateTime(timezone=True), server_default=func.clock_timestamp(), nullable=False
     )
 
     message = relationship("Message")