feat(api,web): tier-based session limits and daily turn usage display

Backend Changes:
- Add tier-based max_sessions config (free: 1, basic: 2, premium: 3, elite: 5)
- Add DELETE /api/v1/sessions/{id} endpoint for hard session deletion
- Cascade delete chat messages when session is deleted
- Add GET /api/v1/usage endpoint for daily turn limit info
- Replace hardcoded TIER_LIMITS with config-based ai_calls_per_day
- Handle unlimited (-1) tier in rate limiter service

Frontend Changes:
- Add inline session delete buttons with HTMX on character list
- Add usage_display.html component showing remaining daily turns
- Display usage indicator on character list and game play pages
- Page refresh after session deletion to update UI state

Documentation:
- Update API_REFERENCE.md with new endpoints and tier limits
- Update API_TESTING.md with session endpoint examples
- Update SESSION_MANAGEMENT.md with tier-based limits

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

api/app/services/chat_message_service.py

@@ -458,6 +458,59 @@ class ChatMessageService:
                         error=str(e))
             raise
 
+    def delete_messages_by_session(self, session_id: str) -> int:
+        """
+        Permanently delete all chat messages associated with a session.
+
+        Used when a session is deleted to clean up associated messages.
+        This is a hard delete - messages are removed from the database entirely.
+
+        Args:
+            session_id: Session ID whose messages should be deleted
+
+        Returns:
+            Number of messages deleted
+
+        Note:
+            This method does not validate ownership because it's called from
+            session_service after ownership has already been validated.
+        """
+        try:
+            # Query all messages with this session_id
+            messages = self.db.list_rows(
+                table_id='chat_messages',
+                queries=[Query.equal('session_id', session_id)],
+                limit=1000  # Handle up to 1000 messages per session
+            )
+
+            deleted_count = 0
+            for message in messages:
+                try:
+                    self.db.delete_document(
+                        collection_id='chat_messages',
+                        document_id=message.id
+                    )
+                    deleted_count += 1
+                except Exception as e:
+                    logger.warning("Failed to delete individual message",
+                                  message_id=message.id,
+                                  session_id=session_id,
+                                  error=str(e))
+                    # Continue deleting other messages
+
+            logger.info("Deleted messages for session",
+                       session_id=session_id,
+                       deleted_count=deleted_count)
+
+            return deleted_count
+
+        except Exception as e:
+            logger.error("Failed to delete messages by session",
+                        session_id=session_id,
+                        error=str(e))
+            # Don't raise - session deletion should still proceed
+            return 0
+
     # Helper Methods
 
     def _update_recent_messages_preview(