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/sqlite_driver.py

@@ -69,15 +69,14 @@ class SQLiteDriver(BaseDriver):
                 ORDER BY name
             """)
             names = [r[0] for r in c.fetchall()]
-        tables = []
-        for name in names:
-            try:
-                with self._cur() as rc:
-                    rc.execute(f'SELECT COUNT(*) FROM "{name}"')
-                    row_count = rc.fetchone()[0]
-            except Exception:
-                row_count = 0
-            tables.append(TableInfo(name=name, schema="main", row_count=row_count))
+            tables = []
+            for name in names:
+                try:
+                    c.execute(f'SELECT COUNT(*) FROM "{name}"')
+                    row_count = c.fetchone()[0]
+                except Exception:
+                    row_count = 0
+                tables.append(TableInfo(name=name, schema="main", row_count=row_count))
         return tables
 
     def get_views(self, database: str = "") -> list:
@@ -164,8 +163,7 @@ class SQLiteDriver(BaseDriver):
 
     def execute_script(self, sql: str) -> list:
         results = []
-        stmts = [s.strip() for s in sql.split(';') if s.strip()]
-        for stmt in stmts:
+        for stmt in self._split_statements(sql):
             try:
                 with self._cur() as c:
                     c.execute(stmt)