feat(zettel): add tenant-safe graph store

[adihex]

Summary

Slice 1 replacing #10.

Dependencies

None.

Behavior

• adds tenant-scoped entities and note-owned graph relations with safe foreign keys/cascades
• atomically and idempotently replaces a note graph after enforcing note ownership
• adds deterministic, tenant-isolated graph traversal with bounded depth and stable deduplication
• preserves existing note editing behavior and migrates the unsafe legacy graph tables when present

Verification

• mise exec -- pnpm --filter zettel run test — 5 files, 36 tests passed
• mise exec -- pnpm build — passed (19 workspace tasks)
• mise exec -- pnpm exec vp check apps/zettel/src/notes/store.ts apps/zettel/src/notes/store.test.ts — passed, 0 warnings/errors
• git diff --check — passed

The repo-wide pnpm check remains blocked by 23 pre-existing formatting issues outside this slice.

[adihex]

Landed on main as part of the consolidated, rebased stack ending at e9bd4bd. Verified this PR has no patches absent from origin/main, so closing the now-redundant stacked PR.