feat(profile): Design user profile schema (ADR)

[arunderwood]

Summary

Write Architecture Decision Record documenting the user profile data model and hybrid storage strategy.

Parent Epic

Part of #309 (User Profiles & Personalization)

Dependencies

• Depends on: epic(admin): Admin Force Refresh with OAuth2 Authentication #203 (Admin Force Refresh) epic complete

Deliverables

• ADR document in docs/adr/004-user-profiles-architecture.md covering:
  • Profile fields: callsign, QTH grid square (Maidenhead), preferred bands, preferred modes
  • Privacy model (what's public vs private)
  • Hybrid storage strategy: LocalStorage for anonymous, DB sync for logged-in
  • API design for profile CRUD operations
  • Sync strategy (LocalStorage ↔ Database)
• Database schema design for user_profiles table
• LocalStorage schema design (JSON structure)

Profile Fields

Field	Type	Description	Example
callsign	string (optional)	Amateur radio callsign	"W1AW"
gridSquare	string (optional)	Maidenhead locator (2-8 chars)	"FN31pr"
preferredBands	string[]	Preferred frequency bands	["20m", "40m", "2m"]
preferredModes	string[]	Preferred operating modes	["SSB", "FT8", "CW"]

Storage Strategy

┌─────────────────────────────────────────────────────────┐
│                    User Preferences                      │
├─────────────────────────────────────────────────────────┤
│  Anonymous User                 Logged-in User          │
│  ┌─────────────┐               ┌─────────────┐          │
│  │ LocalStorage│               │ LocalStorage│          │
│  │ (primary)   │               │ (cache)     │          │
│  └─────────────┘               └──────┬──────┘          │
│                                       │ sync            │
│                                ┌──────▼──────┐          │
│                                │  PostgreSQL │          │
│                                │  (primary)  │          │
│                                └─────────────┘          │
└─────────────────────────────────────────────────────────┘

Acceptance Criteria

• ADR document exists and covers all key decisions
• Database schema documented
• LocalStorage JSON structure documented
• Sync strategy clearly defined