Add multi-provider DNS registry for ACME DNS-01 challenges

Replace hardcoded Cloudflare DNS-01 with a data-driven provider registry
supporting 11 providers (Cloudflare, Route 53, DigitalOcean, Duck DNS,
Hetzner, Vultr, Porkbun, GoDaddy, Namecheap, OVH, Linode). Users can
configure multiple providers with encrypted credentials and select a
default. Per-certificate provider override is supported via providerOptions.

- Add src/lib/dns-providers.ts with provider definitions, credential
  encrypt/decrypt, and Caddy config builder
- Change DnsProviderSettings to multi-provider format with default selection
- Auto-migrate legacy Cloudflare settings on startup (db.ts)
- Normalize old single-provider format on read (getDnsProviderSettings)
- Refactor buildTlsAutomation() to use provider registry
- Add GET /api/v1/dns-providers endpoint for provider discovery
- Add dns-provider settings group to REST API and instance sync
- Replace Cloudflare settings card with multi-provider UI (add/remove
  providers, set default, dynamic credential forms)
- Add 10 DNS provider modules to Caddy Dockerfile
- Update OpenAPI spec, E2E tests, and unit test mocks

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/caddy/Dockerfile

@@ -9,6 +9,16 @@ RUN go install github.com/caddyserver/xcaddy/cmd/xcaddy@latest
 # GOPROXY=direct bypasses the module proxy cache so the latest commit is always fetched
 RUN GOPROXY=direct xcaddy build \
     --with github.com/caddy-dns/cloudflare \
+    --with github.com/caddy-dns/route53 \
+    --with github.com/caddy-dns/digitalocean \
+    --with github.com/caddy-dns/duckdns \
+    --with github.com/caddy-dns/hetzner \
+    --with github.com/caddy-dns/vultr \
+    --with github.com/caddy-dns/porkbun \
+    --with github.com/caddy-dns/godaddy \
+    --with github.com/caddy-dns/namecheap \
+    --with github.com/caddy-dns/ovh \
+    --with github.com/caddy-dns/linode \
     --with github.com/mholt/caddy-l4 \
     --with github.com/fuomag9/caddy-blocker-plugin \
     --with github.com/corazawaf/coraza-caddy/v2 \