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/base.py

@@ -197,6 +197,52 @@ class BaseDriver(ABC):
         """Rename a column via ALTER TABLE."""
         pass
 
+    @staticmethod
+    def _split_statements(sql: str) -> list:
+        """Split SQL text on unquoted semicolons, preserving quoted strings and comments."""
+        stmts, buf = [], []
+        i, n = 0, len(sql)
+        while i < n:
+            ch = sql[i]
+            if ch == '-' and i + 1 < n and sql[i + 1] == '-':
+                end = sql.find('\n', i)
+                end = end + 1 if end != -1 else n
+                buf.append(sql[i:end])
+                i = end
+            elif ch == '/' and i + 1 < n and sql[i + 1] == '*':
+                end = sql.find('*/', i + 2)
+                end = end + 2 if end != -1 else n
+                buf.append(sql[i:end])
+                i = end
+            elif ch in ("'", '"'):
+                q, j = ch, i + 1
+                while j < n:
+                    if sql[j] == '\\':
+                        j += 2
+                    elif sql[j] == q:
+                        j += 1
+                        if j < n and sql[j] == q:
+                            j += 1
+                            continue
+                        break
+                    else:
+                        j += 1
+                buf.append(sql[i:j])
+                i = j
+            elif ch == ';':
+                stmt = ''.join(buf).strip()
+                if stmt:
+                    stmts.append(stmt)
+                buf = []
+                i += 1
+            else:
+                buf.append(ch)
+                i += 1
+        stmt = ''.join(buf).strip()
+        if stmt:
+            stmts.append(stmt)
+        return stmts
+
     @property
     def is_connected(self) -> bool:
         return self._connection is not None