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/config/connections.py

@@ -8,6 +8,9 @@ from pathlib import Path
 from typing import Optional
 
 from app.models.connection_model import ConnectionProfile
+from app.utils.logger import get_logger
+
+_log = get_logger(__name__)
 
 _CONN_FILE = Path.home() / ".dbclient" / "connections.json"
 
@@ -23,8 +26,15 @@ _SERVICE = "DBClient"
 # ── Keyring helpers ──────────────────────────────────────────────────────────
 
 def save_password(profile_id: str, password: str) -> None:
-    if _KEYRING_OK and password:
-        keyring.set_password(_SERVICE, profile_id, password)
+    if not _KEYRING_OK:
+        return
+    if password:
+        try:
+            keyring.set_password(_SERVICE, profile_id, password)
+        except Exception:
+            _log.warning("keyring: failed to save password for %s", profile_id, exc_info=True)
+    else:
+        delete_password(profile_id)
 
 
 def load_password(profile_id: str) -> str:
@@ -32,6 +42,7 @@ def load_password(profile_id: str) -> str:
         try:
             return keyring.get_password(_SERVICE, profile_id) or ""
         except Exception:
+            _log.warning("keyring: failed to load password for %s", profile_id, exc_info=True)
             return ""
     return ""
 
@@ -41,7 +52,7 @@ def delete_password(profile_id: str) -> None:
         try:
             keyring.delete_password(_SERVICE, profile_id)
         except Exception:
-            pass
+            pass  # Already absent is fine
 
 
 # ── Profile CRUD ─────────────────────────────────────────────────────────────