Batch bulk DB writes (execute_batch): unfreeze the discovery step

The live loop froze ~40s every discovery pass. py-spy showed the main thread
stuck in upsert_daily_bars -> PgConnection.executemany, which used psycopg2's
cur.executemany — ONE INSERT round-trip per row. A discovery pass upserts
thousands of daily bars across the universe, so over the LAN Postgres that's
thousands of serial round-trips. While frozen the loop emits nothing and can't
service the trigger/exit cadence.

Switch executemany to psycopg2.extras.execute_batch (page_size=200): many
statements per round-trip, same SQL, same ON CONFLICT upsert semantics.
Benchmarked on the live DB: 771-row upsert 1.87s -> 0.06s (~31x); a real
discovery pass now finishes in one cycle (ingest 6803 rows, no stall).
Helps every bulk writer (daily/minute bars, scanner snapshots).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

storage/db_pg.py

@@ -37,6 +37,7 @@ from typing import Any
 
 import pandas as pd
 import psycopg2
+from psycopg2.extras import execute_batch
 
 SCHEMA_SQL_PATH = Path(__file__).with_name("pg_schema.sql")
 
@@ -138,7 +139,11 @@ class PgConnection:
 
         def go():
             cur = self._conn.cursor()
-            cur.executemany(_translate(sql), seq)
+            # execute_batch sends page_size statements per round-trip; plain
+            # psycopg2 executemany sends ONE per row. Over the remote LAN
+            # Postgres that was ~771 round-trips for a daily-bar upsert and
+            # froze the live loop's discovery step for ~40s each pass.
+            execute_batch(cur, _translate(sql), seq, page_size=200)
             cur.close()
             return self
         return self._run(go)