feat: zero-knowledge encryption Phase 1 — backend opaque + crypto module (WIP)

Backend: server can no longer read user data. All data blobs (medication,
appointment, profile name) are now opaque client-encrypted ciphertext — the
server stores and returns them verbatim, never deserializing the contents.

- Medication: removed MedicationData + flat MedicationResponse; new
  MedicationResponse echoes metadata + encrypted_data blob. Create/update
  accept opaque blobs (whole-blob replace). Update is no longer load-mutate-
  reserialize (server can't read the data).
- Appointment: same opaque treatment; status moved to a top-level document
  field so it remains filterable without decryption.
- Profile: name is now an opaque encrypted blob (name_data/name_iv). Auto-
  created profile on register starts with an empty name; client sets it.
- EncryptedFieldWire type shared across medication/appointment.

Frontend (partial): crypto module using Web Crypto API —
- crypto/keys.ts: double-PBKDF2 derivation (auth secret sent to server +
  encryption key kept in memory); in-memory key store (set/get/clear).
- crypto/cipher.ts: AES-GCM encrypt/decrypt + JSON convenience wrappers.
- crypto/index.ts: re-exports.

NOT YET DONE (frontend integration): auth store key derivation on login/register,
stores decrypt-on-load/encrypt-on-write, types update, UI components wired,
crypto round-trip tests, ADR. This commit is a verified checkpoint — backend
builds clean (21 tests, 0 warnings); frontend crypto module exists but is not
yet wired into the data flow.

backend/src/models/profile.rs

@@ -60,21 +60,21 @@ impl ProfileRepository {
             .await
     }
 
-    /// Update the profile's display name. NOTE: name is currently stored
-    /// plaintext — the model's nameIv/nameAuthTag fields anticipate encryption
-    /// that isn't implemented yet (TODO).
-    pub async fn update_name(
+    /// Update the profile's display name (opaque client-encrypted blob).
+    pub async fn update_encrypted_name(
         &self,
         user_id: &str,
-        name: &str,
+        name_data: &str,
+        name_iv: &str,
+        name_auth_tag: &str,
     ) -> mongodb::error::Result<Option<Profile>> {
         self.collection
             .find_one_and_update(
                 doc! { "userId": user_id },
                 doc! { "$set": {
-                    "name": name,
-                    "nameIv": "",
-                    "nameAuthTag": "",
+                    "name": name_data,
+                    "nameIv": name_iv,
+                    "nameAuthTag": name_auth_tag,
                     "updatedAt": DateTime::now()
                 }},
                 None,