fix: purga de inicio borra api_usage antes de research_sessions

La purga de sesiones >30d fallaba con FOREIGN KEY constraint failed:
api_usage.session_id referencia research_sessions(id) pero nunca se
borraba antes de la sesión padre (con PRAGMA foreign_keys = ON).

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

src/db/database.py

@@ -442,7 +442,7 @@ class ResearchDB:
         )
         session_ids = [row[0] for row in await cursor.fetchall()]
 
-        counts = {"sessions": 0, "sources": 0, "chunks": 0, "outputs": 0}
+        counts = {"sessions": 0, "sources": 0, "chunks": 0, "outputs": 0, "api_usage": 0}
 
         for sid in session_ids:
             await self.db.execute(
@@ -453,6 +453,8 @@ class ResearchDB:
             counts["chunks"] += cur.rowcount
             cur = await self.db.execute("DELETE FROM outputs WHERE session_id = ?", (sid,))
             counts["outputs"] += cur.rowcount
+            cur = await self.db.execute("DELETE FROM api_usage WHERE session_id = ?", (sid,))
+            counts["api_usage"] += cur.rowcount
             cur = await self.db.execute("DELETE FROM sources WHERE session_id = ?", (sid,))
             counts["sources"] += cur.rowcount
             cur = await self.db.execute("DELETE FROM research_sessions WHERE id = ?", (sid,))