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/handlers/appointments.rs

@@ -33,23 +33,12 @@ pub async fn create_appointment(
     let now = SystemTime::now();
     let appointment_id = uuid::Uuid::new_v4().to_string();
 
-    // Pack the request fields into the data blob (camelCase keys).
+    // Zero-knowledge: store the client's opaque encrypted blob verbatim.
     let appointment_data = EncryptedField {
-        data: serde_json::json!({
-            "title": req.title,
-            "provider": req.provider,
-            "appointmentType": req.appointment_type,
-            "dateTime": req.date_time,
-            "location": req.location,
-            "durationMinutes": req.duration_minutes,
-            "reason": req.reason,
-            "notes": req.notes,
-            "status": req.status.unwrap_or_else(|| "upcoming".to_string()),
-        })
-        .to_string(),
-        encrypted: false,
-        iv: String::new(),
-        auth_tag: String::new(),
+        data: req.encrypted_data.data,
+        encrypted: true,
+        iv: req.encrypted_data.iv,
+        auth_tag: req.encrypted_data.auth_tag,
     };
 
     let appointment = crate::models::appointment::Appointment {
@@ -59,6 +48,7 @@ pub async fn create_appointment(
         profile_id: req.profile_id.clone(),
         appointment_data,
         reminders: vec![],
+        status: req.status,
         created_at: DateTime::from_system_time(now),
         updated_at: DateTime::from_system_time(now),
     };