Fix/268 persist undo state

[Ridanshi]

Closes #268

Root cause

undo_last() set action.undone = True on the in-memory dataclass
object only. The ActionLogger had no persistence layer at all — the
entire action list was stored in self._actions: list[ActionRecord] in
process memory and was discarded on every restart. Previously undone
actions reappeared as undoable after restart because there was nothing
to carry the undone state across process lifetimes.

Changes

core/hybrid/action_logger.py

• SQLite persistence via aiosqlite: schema with id, timestamp,
  type, description, domain, session_id, was_guided,
  guidance_confidence, is_undoable, undo_instruction, undone.
• _init_db(): creates the table, then runs ALTER TABLE migrations
  for databases created by an earlier Execra version (adds the three
  undo-related columns if absent, preserving existing action history).
• load(): reads all rows from SQLite ordered by timestamp and
  reconstructs the in-memory list — including every undone flag. Call
  this once at startup so undo history survives restarts.
• record_action(): INSERT into SQLite first, then append to the
  in-memory list, so a failed DB write never leaves the two stores out
  of sync.
• undo_last(): sets action.undone = True in memory and
  executes UPDATE action_log SET undone = 1 WHERE id = ? so the flag
  is durable across restarts.
• replay_session(): now excludes actions where undone=True so
  the replay reflects only the committed state of the session.
• clear_session(): DELETE FROM action_log WHERE session_id = ?
  in addition to filtering the in-memory list.

api/main.py

Added await action_logger.load() to the FastAPI startup event so
the in-memory list is populated from the database before the first
request is served.

api/routes/actions.py

create_action and undo_last_action converted to async def with
await on record_action() / undo_last().

Tests

tests/unit/test_action_logger.py — full rewrite with tmp_path-isolated
SQLite databases:

Test	What it verifies
test_undo_state_survives_restart	Undo an action, create a fresh ActionLogger against the same DB, call load() — action is still marked undone, undo_last() returns None
test_multiple_undos_survive_restart	Undo two of three actions, restart, verify both remain undone
test_undo_last_updates_undone_column_in_database	Direct DB query confirms undone=1 after undo_last()
test_memory_and_database_stay_in_sync_after_undo	Both in-memory and DB show undone=True
test_replay_session_excludes_undone_actions	Undone actions absent from replay
test_replay_session_respects_undone_state_after_restart	Replay after load() still excludes undone actions

23/23 tests pass. Black, isort, flake8 clean.