feat: versioned store-migration policy for persisted data (v5 step 2)

Third v5.0.0 roadmap item: define and implement a migration policy for the
on-device persisted store (tracked domains, history/snapshots, audits,
workflows, monitoring, settings), so data upgrades cleanly across app versions
instead of relying on a one-shot marker.

- DataMigrationService is reworked from a single boolean marker
  (`data.migrations.v3_4_0`) into a versioned runner keyed by an integer store
  schema version (`data.storeSchemaVersion`). It runs each step once in
  ascending order up to `currentStoreSchemaVersion`, stamping the version as it
  goes. Adding a future migration is now a `case N:` plus a version bump.

  Policy guarantees, all covered by tests:
  - Forward-only and idempotent; every step must be safe on an empty/older store.
  - Never downgrades: a store written by a newer build (higher version) is left
    byte-for-byte untouched.
  - Pre-versioning installs are handled: a set legacy boolean marker reads as
    "already at v1", so the v1 normalization never re-runs for them.

  v1 is the existing normalization pass (dedup + drop the legacy `watchedDomains`
  key + sanitize monitoring settings), now expressed as migration step 1.

- Docs/data-migration.md documents the persisted surface, the two independent
  version lines (store vs. backup export), when to use lenient decoding vs. a
  migration step, the runner contract, an "adding a migration" checklist, and
  backup-import compatibility. Linked from the README.

- DataMigrationServiceTests: 6 tests over legacy fixtures — fresh-store stamping,
  legacy `watchedDomains` migration + key drop, in-place dedup of the stored
  blob, idempotence, legacy-marker-as-v1, and the no-downgrade guard. Full unit
  suite: 58 passing.

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