fix: reject empty prose on turn submit

Empty submission was producing a blank user_turn event in the log and
firing the LLM stream anyway — the bot would invent a response from the
kickoff context alone, producing a monologue with no user input. Two-
layer fix:

- Browser: add `required` to the prose textarea in chat.html so the
  form refuses to submit empty.
- Server: 400 in post_turn when prose.strip() is empty. Defense in
  depth — if a client bypasses the textarea attribute (custom UI,
  curl, etc.), the server still rejects.

Verified live: POST with empty body returns 400; POST with whitespace-
only returns 400; chat shell renders the textarea with required.
Full suite: 168 passed.

chat/templates/chat.html

@@ -26,7 +26,7 @@
   </section>
 
   <form class="turn-input" method="post" action="/chats/{{ chat.id }}/turns">
-    <textarea name="prose" rows="3" placeholder="What do you say or do?"></textarea>
+    <textarea name="prose" rows="3" placeholder="What do you say or do?" required></textarea>
     <button type="submit">Send</button>
   </form>