feat: P0/P1 fixes — backup, export, gc, performance, auto-consolidation, encryption

P0 fixes:
- Add `vestige backup <path>` — full DB copy with WAL checkpoint flush
- Add `vestige export --format json|jsonl [--tags] [--since] <path>` —
  paginated memory export with tag/date filtering
- Add `vestige gc --min-retention 0.1 [--max-age-days] [--dry-run] [--yes]`
  — bulk cleanup of stale memories with safety prompts
- Fix apply_decay() scaling: batched pagination (500 rows/batch) with
  explicit transactions instead of loading all nodes into memory
- Fix hidden MCP resources: memory://insights and memory://consolidation-log
  now listed in resources/list (were implemented but undiscoverable)

P1 fixes:
- Add auto-consolidation on server startup: FSRS-6 decay runs in background
  after 2s delay, only if last consolidation was >6 hours ago
- Add encryption at rest via SQLCipher feature flag: use --features encryption
  with VESTIGE_ENCRYPTION_KEY env var (bundled-sqlite and encryption are
  mutually exclusive)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -3522,6 +3522,7 @@ dependencies = [
  "colored",
  "directories",
  "rmcp",
+ "rusqlite",
  "serde",
  "serde_json",
  "tempfile",