Update documentation for hybrid config and Gmail API vs SMTP delivery

- README.md: document hybrid config model, Gmail API vs SMTP comparison,
  update architecture diagram, replace Postmarkapp with Apprise
- ARCHITECTURE.md: add Gmail API vs SMTP comparison table, document
  ConfigService and AppSetting, add settings/gmail API endpoints
- CHANGELOG.md: add entries for database-backed config and documentation
- TODO.md: update progress (59% coverage, production readiness 30%)
- .env.example: document bootstrap vs database-managed settings

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/ce88d4a8-d8c2-49b3-95a1-30592105a769

.env.example

@@ -13,20 +13,26 @@ POP3_ACCOUNT_1_USE_SSL=true
 # POP3_ACCOUNT_2_PASSWORD=another_password
 # POP3_ACCOUNT_2_USE_SSL=true
 
-# Gmail/SMTP Configuration
+# ── Bootstrap Settings (always from env, never from database) ──────
+# DATABASE_URL=postgresql+asyncpg://user:password@localhost:5432/pop3_forwarder
+# SECRET_KEY=<generate with: python -c 'import secrets; print(secrets.token_urlsafe(32))'>
+# ENCRYPTION_KEY=<generate with: python -c 'import secrets; print(secrets.token_urlsafe(32))'>
+
+# ── Settings below can also be managed via the database ────────────
+# Use the admin API (PUT /api/v1/settings/{key}) to store them in
+# PostgreSQL. Database values take precedence over env vars.
+
+# Gmail/SMTP Configuration (fallback delivery method)
 SMTP_HOST=smtp.gmail.com
 SMTP_PORT=587
 SMTP_USER=your-email@gmail.com
 SMTP_PASSWORD=your-app-password
 SMTP_USE_TLS=true
 
-# Gmail destination
-GMAIL_DESTINATION=your-email@gmail.com
-
-# Postmarkapp for Error Notifications
-POSTMARK_API_TOKEN=your-postmark-api-token
-POSTMARK_FROM_EMAIL=errors@yourdomain.com
-POSTMARK_TO_EMAIL=admin@yourdomain.com
+# Gmail API Configuration (preferred delivery method)
+# GOOGLE_CLIENT_ID=your-google-client-id
+# GOOGLE_CLIENT_SECRET=your-google-client-secret
+# GMAIL_API_ENABLED=true
 
 # Scheduling
 CHECK_INTERVAL_MINUTES=5