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.py

@@ -25,8 +25,18 @@ class Message(Base):
     content: Mapped[str | None] = mapped_column(Text)
     image_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("message_images.id"))
     file_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("message_files.id"))
+    # clock_timestamp(), not now()/func.now() -- the WS handler (ws/chat.py)
+    # shares one AsyncSession for a whole connection's lifetime, and a
+    # read-only op (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 the *transaction's* start
+    # time in that case, not the actual INSERT's -- confirmed live to be the
+    # actual cause of #45 (a reply sent well after an idle read-only period
+    # got timestamped to when that period started, sorting it before
+    # messages that were genuinely sent earlier). clock_timestamp() always
+    # reflects the real moment of execution regardless of transaction age.
     created_at: Mapped[datetime] = mapped_column(
-        DateTime(timezone=True), server_default=func.now(), index=True, nullable=False
+        DateTime(timezone=True), server_default=func.clock_timestamp(), index=True, nullable=False
     )
     edited_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))