feat: profile sharing via X25519 envelope (Phase B, #3)

An owner can now share a profile with another account; the recipient
reads the profile's metadata AND its data (medications, appointments,
health stats) under their own login. The server stays a blind store:
sharing uses an X25519 envelope — the owner wraps the profile DEK to
the recipient's identity public key via ECDH (fresh ephemeral key per
share), the recipient unwraps it with their identity private key.

Backend:
- models/profile_share.rs: ProfileShare + ProfileShareRepository
  (find_for_recipient, find_for_profile, find_active [checks active +
  expiry], delete, upsert). Indexed on (profileId, recipientUserId) and
  recipientUserId.
- handlers/profile_share.rs: POST/GET /profiles/:id/shares,
  DELETE /profiles/:id/shares/:recipient, GET /profiles/shared-with-me,
  GET /users/public-key (public), and authorize_profile_read — the
  share-gate that admits owner OR active-share recipient.
- The share-gate is wired into list/get for medications, appointments,
  and health stats: when profile_id is specified, resolve the owner via
  the gate and query as them. Data repos stay ownership-scoped.
- Removed the legacy Share system (ADR Open Q5): models/{share,
  permission}.rs, handlers/{shares,permissions}.rs, middleware/
  permission.rs (dead), the shares collection field + methods in
  mongodb_impl.rs, the shares index, and the 5 /api/shares +
  /api/permissions routes.
- share_tests.rs: full owner→recipient→revoke flow, ownership
  isolation, share-to-self/nonexistent/keyless rejections, expired
  share treated as absent.

Frontend:
- crypto/keys.ts: wrapProfileDekToRecipient /
  unwrapProfileDekFromShare (ECDH envelope, ephemeral key per share).
- useProfileStore: loadSharedWithMe (unwrap each share's DEK with the
  identity private key), shareProfile, revokeShare, loadProfileShares.
  Shared profiles merge into the list with is_shared=true.
- ProfileSwitcher: shows shared profiles with a 'shared' chip.
- ProfileSharing (new) + ProfileEditor: owner UI to add a recipient by
  email and revoke; shared profiles render read-only with owner info.
- ECDH round-trip test (owner wraps, recipient unwraps, stranger can't).

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/Docker in this
sandbox); CI will run them — I'll fix any failures immediately.

Phases C (hard revoke / re-key) and D (graduation) remain. Refs #3.

backend/src/db/init.rs

@@ -98,15 +98,30 @@ impl DatabaseInitializer {
             println!("✓ Created appointments collection");
         }
 
-        // Create shares collection and index
+        // Create profile_shares collection and indexes (Phase B — replaces the
+        // legacy shares collection). Lookups are by (profileId, recipientUserId)
+        // and by recipientUserId alone (for /profiles/shared-with-me).
         {
-            let collection: Collection<mongodb::bson::Document> = self.db.collection("shares");
+            let collection: Collection<mongodb::bson::Document> =
+                self.db.collection("profile_shares");
 
-            let index = IndexModel::builder().keys(doc! { "familyId": 1 }).build();
+            let pair_index = IndexModel::builder()
+                .keys(doc! { "profileId": 1, "recipientUserId": 1 })
+                .build();
+            match collection.create_index(pair_index, None).await {
+                Ok(_) => println!("✓ Created index on profile_shares (profileId, recipientUserId)"),
+                Err(e) => println!("Warning: Failed to create profile_shares pair index: {}", e),
+            }
 
-            match collection.create_index(index, None).await {
-                Ok(_) => println!("✓ Created index on shares.familyId"),
-                Err(e) => println!("Warning: Failed to create index on shares.familyId: {}", e),
+            let recipient_index = IndexModel::builder()
+                .keys(doc! { "recipientUserId": 1 })
+                .build();
+            match collection.create_index(recipient_index, None).await {
+                Ok(_) => println!("✓ Created index on profile_shares.recipientUserId"),
+                Err(e) => println!(
+                    "Warning: Failed to create profile_shares recipient index: {}",
+                    e
+                ),
             }
         }