feat: /watch --at HH:MM — hora absoluta en Europe/Madrid

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/db/database.py

@@ -378,13 +378,16 @@ class ResearchDB:
 
     # --- Watched Topics ---
 
-    async def add_watch(self, topic: str, chat_id: int, interval_hours: int) -> int:
+    async def add_watch(self, topic: str, chat_id: int, interval_hours: int,
+                        next_run_at: Optional[float] = None) -> int:
         now = time.time()
+        if next_run_at is None:
+            next_run_at = now + interval_hours * 3600
         cursor = await self.db.execute(
             """INSERT OR REPLACE INTO watched_topics
                (topic, chat_id, interval_hours, next_run_at, created_at)
                VALUES (?, ?, ?, ?, ?)""",
-            (topic, chat_id, interval_hours, now + interval_hours * 3600, now)
+            (topic, chat_id, interval_hours, next_run_at, now)
         )
         await self.db.commit()
         return cursor.lastrowid