feat(backend): Complete Phase 2.5 - Access Control Implementation

Implement comprehensive permission-based access control system with share management.

Features:
- Permission model (Read, Write, Admin)
- Share model for resource sharing between users
- Permission middleware for endpoint protection
- Share management API endpoints
- Permission check endpoints
- MongoDB repository implementations for all models

Files Added:
- backend/src/db/permission.rs - Permission repository
- backend/src/db/share.rs - Share repository
- backend/src/db/user.rs - User repository
- backend/src/db/profile.rs - Profile repository
- backend/src/db/appointment.rs - Appointment repository
- backend/src/db/family.rs - Family repository
- backend/src/db/health_data.rs - Health data repository
- backend/src/db/lab_result.rs - Lab results repository
- backend/src/db/medication.rs - Medication repository
- backend/src/db/mongodb_impl.rs - MongoDB trait implementations
- backend/src/handlers/permissions.rs - Permission API handlers
- backend/src/handlers/shares.rs - Share management handlers
- backend/src/middleware/permission.rs - Permission checking middleware

API Endpoints:
- GET /api/permissions/check - Check user permissions
- POST /api/shares - Create new share
- GET /api/shares - List user shares
- GET /api/shares/:id - Get specific share
- PUT /api/shares/:id - Update share
- DELETE /api/shares/:id - Delete share

Status: Phase 2.5 COMPLETE - Building successfully, ready for production

backend/src/handlers/mod.rs

@@ -1,13 +1,25 @@
-### /home/asoliver/desarrollo/normogen/./backend/src/handlers/mod.rs
-```rust
-1: pub mod auth;
-2: pub mod users;
-3: pub mod health;
-4: 
-5: pub use auth::*;
-6: pub use users::*;
-7: pub use health::*;
-```
-
+pub mod auth;
+pub mod health;
+pub mod users;
 pub mod shares;
-pub use shares::*;
+pub mod permissions;
+
+// Auth handlers
+pub use auth::{
+    register, login, recover_password,
+};
+
+// User handlers
+pub use users::{
+    get_profile, update_profile, delete_account,
+    get_settings, update_settings, change_password,
+};
+
+// Health handlers
+pub use health::{health_check, ready_check};
+
+// Share handlers
+pub use shares::{create_share, list_shares, get_share, update_share, delete_share};
+
+// Permission handlers
+pub use permissions::check_permission;