persist perplexity per assistant message; display on loaded convos

db.py

@@ -123,6 +123,7 @@ def init_db():
         CREATE TABLE IF NOT EXISTS messages (
             id INTEGER PRIMARY KEY AUTOINCREMENT, conversation_id TEXT NOT NULL,
             role TEXT NOT NULL, content TEXT NOT NULL, created_at TEXT NOT NULL,
+            perplexity REAL,
             FOREIGN KEY (conversation_id) REFERENCES conversations(id) ON DELETE CASCADE
         )
     """)
@@ -164,6 +165,11 @@ def init_db():
     except Exception:
         pass
 
+    try:
+        conn.execute("ALTER TABLE messages ADD COLUMN perplexity REAL")
+    except Exception:
+        pass
+
     if not conn.execute("SELECT id FROM profile WHERE id = 1").fetchone():
         now = datetime.now(timezone.utc).isoformat()
         conn.execute("INSERT INTO profile (id, content, updated_at) VALUES (1, ?, ?)", (DEFAULT_PROFILE, now))