Email Linking System Documentation

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+# Email Linking System
+
+## Overview
+
+The email linking system allows users to authenticate with multiple email addresses that all resolve to the same account. This is particularly useful for developers with both work (`@aethex.dev`) and personal email addresses.
+
+## Architecture
+
+### Database Tables
+
+#### `user_email_links`
+Links multiple email addresses to a single user account.
+
+```sql
+- id: UUID (primary key)
+- user_id: UUID (FK to user_profiles.user_id)
+- email: TEXT (unique)
+- is_primary: BOOLEAN (marks the main email)
+- verified_at: TIMESTAMP
+- linked_at: TIMESTAMP
+- created_at: TIMESTAMP
+- updated_at: TIMESTAMP
+```
+
+#### `user_profiles` (additions)
+```sql
+- primary_email: TEXT (the primary email for the account)
+- is_dev_account: BOOLEAN (marks if this is a developer account)
+- merged_to_user_id: UUID (if this account was merged, points to the primary account)
+```
+
+## User Flows
+
+### Flow 1: Login with Linked Email
+
+```
+User attempts login with: mrpiglr@gmail.com
+↓
+SignIn catches "invalid credentials" error
+↓
+Calls /api/user/resolve-linked-email with email
+↓
+Check user_email_links table
+↓
+Found! Returns primary email: mrpiglr@aethex.dev
+↓
+Retry SignIn with primary email
+↓
+Success - user logged in
+```
+
+### Flow 2: Link Additional Email (Settings)
+
+```
+User clicks "Link Email" in settings
+↓
+Enters secondary email: mrpiglr@gmail.com
+↓
+Verification sent to that email
+↓
+User verifies via link
+↓
+Email added to user_email_links with is_primary=false
+↓
+UI shows: "Also logged in as mrpiglr@gmail.com"
+```
+
+## API Endpoints
+
+### POST `/api/user/resolve-linked-email`
+
+Resolve a linked email to its primary email address.
+
+**Request:**
+```json
+{
+  "email": "mrpiglr@gmail.com"
+}
+```
+
+**Response (linked email):**
+```json
+{
+  "primaryEmail": "mrpiglr@aethex.dev",
+  "linkedFrom": "mrpiglr@gmail.com",
+  "userId": "uuid-here"
+}
+```
+
+**Response (non-linked email):**
+```json
+{
+  "primaryEmail": "some@email.com"
+}
+```
+
+### POST `/api/user/link-email`
+
+Link two existing user accounts by merging one into the other.
+
+**Request:**
+```json
+{
+  "primaryEmail": "mrpiglr@aethex.dev",
+  "linkedEmail": "mrpiglr@gmail.com"
+}
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Successfully linked mrpiglr@gmail.com to mrpiglr@aethex.dev",
+  "targetUserId": "primary-user-uuid",
+  "sourceUserId": "linked-user-uuid"
+}
+```
+
+### POST `/api/user/link-mrpiglr-accounts` (Admin Only)
+
+Special endpoint to link the mrpiglr accounts.
+
+**Request:**
+```bash
+curl -X POST https://aethex.dev/api/user/link-mrpiglr-accounts \
+  -H "Authorization: Bearer mrpiglr-admin-token" \
+  -H "Content-Type: application/json"
+```
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Successfully linked mrpiglr@aethex.dev and mrpiglr@gmail.com",
+  "primaryEmail": "mrpiglr@aethex.dev",
+  "linkedEmail": "mrpiglr@gmail.com",
+  "note": "Both emails can now log in and will access the same account."
+}
+```
+
+## Implementation Details
+
+### Authentication Resolution
+
+When a user tries to sign in with a linked email:
+
+1. **First attempt**: Try with provided email (normal auth flow)
+2. **If it fails with "invalid credentials"**:
+   - Check `/api/user/resolve-linked-email`
+   - If email is linked, get primary email
+   - **Retry with primary email**
+   - If successful, user is logged in
+
+This is transparent to the user - they can log in with any linked email.
+
+### Data Transfer on Merge
+
+When two accounts are merged:
+
+1. **Achievements**: Transferred to primary account
+2. **Creator Profile**: Transferred or preserved if not duplicate
+3. **Applications**: Transferred to primary account
+4. **Discord Links**: Transferred (avoiding duplicates)
+5. **Web3 Wallets**: Transferred (avoiding duplicates)
+6. **Email Links**: Both emails added to user_email_links table
+7. **Profile**: Source profile marked as `merged_to_user_id`
+
+### For Developers (Dev Accounts)
+
+Developer accounts with `@aethex.dev` email:
+- `is_dev_account = true`
+- `primary_email = "@aethex.dev email"`
+- Public profile shows work email
+- Can link personal email for authentication convenience
+
+## Usage Examples
+
+### Link mrpiglr Accounts
+
+```bash
+# Admin endpoint (one-time setup)
+curl -X POST https://aethex.dev/api/user/link-mrpiglr-accounts \
+  -H "Authorization: Bearer mrpiglr-admin-token" \
+  -H "Content-Type: application/json"
+```
+
+### Link Any Two Accounts
+
+```bash
+# Generic link endpoint
+curl -X POST https://aethex.dev/api/user/link-email \
+  -H "Content-Type: application/json" \
+  -d '{
+    "primaryEmail": "user@aethex.dev",
+    "linkedEmail": "user@gmail.com"
+  }'
+```
+
+### Check Email Resolution
+
+```bash
+curl -X POST https://aethex.dev/api/user/resolve-linked-email \
+  -H "Content-Type: application/json" \
+  -d '{"email": "user@gmail.com"}'
+```
+
+## Database Migration
+
+Applied via: `code/supabase/migrations/20250206_add_email_linking.sql`
+
+Includes:
+- `user_email_links` table with RLS policies
+- `get_primary_user_by_email()` function
+- Columns on `user_profiles` for dev accounts and primary email
+- Proper indexes for performance
+
+## Future Enhancements
+
+1. **Email Verification**: Add verification flow before linking new emails
+2. **Settings UI**: Add "Linked Emails" section in user settings
+3. **Email Change Request**: Allow users to change primary email
+4. **Admin Dashboard**: View all linked emails for a user
+5. **Notification**: Email both addresses when a new email is linked