feat(profile): Implement user profile persistence for logged-in users

[arunderwood]

Summary

Add database persistence for user profiles, enabling cross-device sync for logged-in users.

Parent Epic

Part of #309 (User Profiles & Personalization)

Dependencies

• Depends on: LocalStorage preferences issue

Deliverables

Database Schema

• Create Liquibase migration for user_profiles table:

  CREATE TABLE user_profiles (
    id BIGSERIAL PRIMARY KEY,
    user_id BIGINT NOT NULL REFERENCES app_users(id) UNIQUE,
    callsign VARCHAR(10),
    grid_square VARCHAR(8),
    preferred_bands TEXT[],
    preferred_modes TEXT[],
    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
  );
  
  CREATE INDEX idx_user_profiles_user_id ON user_profiles(user_id);

Backend

• Create UserProfileEntity.java
• Create UserProfileRepository.java
• Create ProfileEndpoint.java with @BrowserCallable:

  @BrowserCallable
  public class ProfileEndpoint {
      @PermitAll  // Any authenticated user
      public Optional<UserProfile> getProfile() { ... }
  
      @PermitAll
      public UserProfile updateProfile(ProfileUpdateRequest request) { ... }
  }

Sync Strategy

• On login: Merge LocalStorage → DB (DB wins for conflicts, or use "last modified" timestamp)
• On login: Populate LocalStorage from DB if LocalStorage is empty
• Update usePreferences hook to detect auth state and sync

Testing

• Unit tests for ProfileEndpoint
• Integration tests for sync behavior
• Test conflict resolution

Sync Flow

Login detected
    │
    ├─► LocalStorage empty?
    │   └─► YES: Fetch from DB → populate LocalStorage
    │   └─► NO: Compare timestamps
    │           └─► DB newer: Update LocalStorage
    │           └─► Local newer: Update DB
    │
    └─► Subscribe to preference changes → sync to DB

Acceptance Criteria

• Logged-in user's preferences sync to database
• Preferences restored on login from different device
• LocalStorage acts as cache for logged-in users
• Logout does NOT clear LocalStorage (anonymous persistence)