Add persistent settings system with database backend and comprehensive UI

- New Setting ORM model (key-value store with category, value_type, audit fields)
- Alembic migration to create the settings table
- Settings API endpoints: GET/PUT /api/v1/settings/{key}, GET /api/v1/settings (list+filter), POST /api/v1/settings/bulk
- Default seeding (17 sensible defaults across general/dmarc/dns/cloudflare/notifications categories)
- Secret redaction for cloudflare.api_token and notifications.smtp_password
- Updated settings.html: General, DMARC Policy Defaults, DNS Resolver, Cloudflare Integration, Email Notifications sections
- All forms wired to the API via Alpine.js with flash feedback
- 12 new tests for the settings model and endpoints

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/19dbc6cd-07cb-406e-b3b6-411f7721f737

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/api/api_v1/api.py

@@ -1,6 +1,15 @@
 from fastapi import APIRouter
 
-from app.api.api_v1.endpoints import domains, health, imap, mail_sources, reports, setup, stats
+from app.api.api_v1.endpoints import (
+    domains,
+    health,
+    imap,
+    mail_sources,
+    reports,
+    settings,
+    setup,
+    stats,
+)
 
 api_router = APIRouter()
 
@@ -12,3 +21,4 @@ api_router.include_router(setup.router, prefix="/setup", tags=["setup"])
 api_router.include_router(imap.router, prefix="/imap", tags=["imap"])
 api_router.include_router(stats.router, prefix="/stats", tags=["stats"])
 api_router.include_router(mail_sources.router, prefix="/mail-sources", tags=["mail-sources"])
+api_router.include_router(settings.router, prefix="/settings", tags=["settings"])