feat: add Terms & Conditions checkbox to signup (refs #4)

- Add terms_agreements table for legal audit trail
- Create terms-agreement feature capsule with repository
- Modify signup to create terms agreement atomically
- Add checkbox with PDF link to SignupForm
- Capture IP, User-Agent, terms version, content hash
- Update CLAUDE.md documentation index

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/terms-agreement/README.md

@@ -0,0 +1,46 @@
+# Terms Agreement Feature
+
+Stores legal audit trail for Terms & Conditions acceptance at user signup.
+
+## Purpose
+
+Provides comprehensive legal compliance by capturing:
+- **User consent**: Proof that user agreed to T&C before account creation
+- **Audit fields**: IP address, user agent, timestamp, terms version, content hash
+
+## Data Flow
+
+1. User checks T&C checkbox on signup form
+2. Frontend sends `termsAccepted: true` with signup request
+3. Controller extracts IP (from X-Forwarded-For header), User-Agent
+4. Service creates user profile and terms agreement atomically in transaction
+5. Terms agreement record stores audit fields for legal compliance
+
+## Database Schema
+
+| Column | Type | Description |
+|--------|------|-------------|
+| id | UUID | Primary key |
+| user_id | VARCHAR | Auth0 user ID |
+| agreed_at | TIMESTAMPTZ | UTC timestamp of agreement |
+| ip_address | VARCHAR(45) | Client IP (supports IPv6) |
+| user_agent | TEXT | Browser/client user agent |
+| terms_version | VARCHAR | Version string (e.g., v2026-01-03) |
+| terms_url | VARCHAR | URL path to PDF |
+| terms_content_hash | VARCHAR(64) | SHA-256 hash of PDF content |
+
+## Files
+
+| Path | Purpose |
+|------|---------|
+| `migrations/001_create_terms_agreements.sql` | Database table creation |
+| `data/terms-agreement.repository.ts` | Data access layer |
+| `domain/terms-agreement.types.ts` | TypeScript interfaces |
+| `domain/terms-config.ts` | Static terms version and hash |
+| `index.ts` | Feature exports |
+
+## Invariants
+
+- Every user in `user_profiles` has exactly one record in `terms_agreements`
+- `agreed_at` is always stored in UTC
+- `terms_content_hash` matches SHA-256 of PDF at signup time