fix(dim_freshness): check WAL mtime to avoid false-stale freshness reports

[jsherlockppb]

Problem

SQLite in WAL mode writes new data to session-store.db-wal first. The main .db file is only updated when a checkpoint occurs (periodically, not on every write).

The dim_freshness check was using os.path.getmtime(DB_PATH) — the main file only. This meant that even with active sessions being recorded, the freshness dimension would report AMBER or RED because the main file's mtime hadn't changed.

Fix

Take the most recent mtime across both the main DB and the WAL file:

wal_path = DB_PATH + "-wal"
mtime = max(
    os.path.getmtime(DB_PATH),
    os.path.getmtime(wal_path) if os.path.exists(wal_path) else 0,
)

This correctly reflects when data was last written, regardless of checkpoint timing.

Tests added

New file: src/session_recall/tests/test_dim_freshness.py (5 tests)

Test	Scenario
test_green_when_db_recently_modified	Recent DB → GREEN
test_red_when_db_missing	No DB file → RED
test_wal_mtime_used_when_newer_than_db	WAL is fresh, DB is 100h stale → GREEN (key regression test)
test_db_mtime_used_when_no_wal	No WAL present → falls back to DB mtime
test_stale_when_both_db_and_wal_are_old	Both DB and WAL are old → RED

Verified locally

• Freshness now reports 0.0h old (GREEN) after active sessions, vs previously showing 68h old (AMBER)
• All 95 tests pass (90 existing + 5 new)