Fix medium/low severity issues: script splitter, EXPLAIN, keyring, imports

execute_script() semicolon splitter (all drivers)
- Add BaseDriver._split_statements() that tracks single/double-quoted
  strings and -- / /* */ comments so semicolons inside procedure bodies
  are not treated as statement boundaries.
- Replace the naive sql.split(';') in all four drivers with this helper.
- MSSQL execute_script() also pre-splits on GO (case-insensitive, own
  line) so scripts pasted from SSMS work correctly.

MSSQL EXPLAIN
- SET SHOWPLAN_TEXT ON/execute/SET SHOWPLAN_TEXT OFF must be separate
  execute() calls; pyodbc rejects multiple statements in one call.
  Hold self._lock for the entire sequence to keep session state atomic.

Keyring (connections.py)
- Log warnings (with traceback) on load/save failures instead of
  silently returning "".
- save_password() now calls delete_password() when password is empty,
  so clearing a saved password actually removes the old keyring entry
  rather than leaving a stale one behind.

SQLite get_tables() O(N) lock acquisitions
- Run all COUNT(*) queries inside the same `with self._cur() as c:`
  block, reducing N+1 lock acquisitions to 1.

Unused import: remove psycopg2.extras from postgres_driver.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/drivers/postgres_driver.py

@@ -1,6 +1,5 @@
 """PostgreSQL driver implementation using psycopg2."""
 import psycopg2
-import psycopg2.extras
 from contextlib import contextmanager
 from typing import Optional
 from app.drivers.base import (
@@ -230,9 +229,8 @@ class PostgreSQLDriver(BaseDriver):
 
     def execute_script(self, sql: str) -> list:
         results = []
-        stmts = [s.strip() for s in sql.split(';') if s.strip()]
         with self._cur() as c:
-            for stmt in stmts:
+            for stmt in self._split_statements(sql):
                 try:
                     c.execute(stmt)
                     if c.description: