feat: hard revoke / re-key (Phase C, #3)

An owner can now rotate a profile's DEK — generating a fresh key,
re-encrypting all the profile's data under it (client-side), and
re-wrapping to the owner share + kept recipients. A revoked recipient's
cached old DEK stops working, closing the soft-revoke window the ADR
requires for graduation / suspected compromise.

The server stays a blind store: it sees opaque old→new ciphertext blobs
flow through, never the DEK or plaintext. No backend crypto.

Backend:
- ProfileRepository::update_wrapped_dek — rotate the owner share
  (account-wrapped profile DEK), owner-scoped.
- ProfileShareRepository::find_active_for_profile +
  delete_for_profile_excluding — list current recipients and hard-delete
  omitted ones.
- POST /api/profiles/:id/rekey: validates each submitted envelope
  against existing active shares (no smuggling new recipients in via
  rekey), rotates the owner share, upserts recipient envelopes with
  fresh ephemeral ECDH keys, hard-deletes omitted recipients.
- rekey_tests.rs: rotation, hard-revoke-from-recipient-view, no-share
  rejection, non-owner rejection, revoke-all.

Frontend:
- useProfileStore.rekeyProfile: fetches all profile data, re-encrypts
  each row under a new DEK (resume-safe — rows already on the new DEK
  are skipped), builds fresh ECDH envelopes per kept recipient, commits
  via POST /rekey, swaps the in-memory DEK.
- ProfileSharing: 'Rotate encryption key (hard revoke)' action with a
  strong confirmation dialog explaining the cost and the resume-safe
  retry.

Best-effort + resumable retry (per design decision); no server-side
write lock.

Verification: backend cargo build/clippy (-D warnings)/fmt clean, tests
compile (integration tests run in CI — Mongo is fixed there). Frontend
tsc clean, 31/31 tests pass.

⚠️ Backend integration tests not run locally (no Mongo in this sandbox);
CI will run them — I'll fix any failures.

Admin-initiated rekey (ADR §5c reserves the permission) is out of scope
— handler is owner-only until admin shares are creatable in Phase D.
Refs #3.

backend/src/app.rs

@@ -66,6 +66,8 @@ pub fn build_app(state: AppState) -> Router {
             get(handlers::list_shares).post(handlers::create_share),
         )
         .route("/api/profiles/:id/shares/:recipient", delete(handlers::delete_share))
+        // Hard revoke / rotate the profile DEK (Phase C). Owner-only.
+        .route("/api/profiles/:id/rekey", post(handlers::rekey_profile))
         // Session management (Phase 2.6)
         .route("/api/sessions", get(handlers::get_sessions))
         .route("/api/sessions/:id", delete(handlers::revoke_session))