Add DuckDNS as a free DNS provider option (#124)

DuckDNS's API is smaller than the other providers' (no list/read API,
no arbitrary sub-records, one A/AAAA + one TXT record per domain), so
domains are entered by the operator instead of auto-discovered, and
getRecords reads from public DNS since there's nothing else to query.
Documented as a free option in the README and DNS provider docs.

docs/api.md

@@ -687,7 +687,7 @@ curl -H "auth-token: your-token-here" \
 ```
 
 **Responses:**
-- `200` `{"results": [{"name": "Cloudflare", "fields": {...}}, {"name": "DigitalOcean", ...}, ...]}`
+- `200` `{"results": [{"name": "Cloudflare", "fields": {...}}, {"name": "DigitalOcean", ...}, {"name": "PorkBun", ...}, {"name": "DuckDns", ...}]}`
 
 ### Create DNS Provider
 
@@ -722,6 +722,21 @@ curl -H "Content-Type: application/json" \
   https://proxy-host.com/api/dns
 ```
 
+**DuckDNS (free):**
+```bash
+curl -H "Content-Type: application/json" \
+  -H "auth-token: your-token-here" \
+  -X POST \
+  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "domains": "myhost,myhost2"}' \
+  https://proxy-host.com/api/dns
+```
+
+`domains` is a comma-separated list of the subdomains you've registered at
+[duckdns.org](https://www.duckdns.org) (e.g. `myhost` for
+`myhost.duckdns.org`), since DuckDNS has no API to list them for you.
+DuckDNS only supports one A/AAAA record and one TXT record per domain (no
+arbitrary sub-records) — enough for dynamic DNS and DNS-01 wildcard certs.
+
 **Responses:**
 - `200` `{"message": "\"provider-id\" added.", ...}`
 - `422` Validation error or invalid API credentials

docs/architecture.md

@@ -45,6 +45,7 @@ The proxy system consists of three main components working together to provide h
                           │  - Cloudflare        │
                           │  - DigitalOcean      │
                           │  - PorkBun           │
+                          │  - DuckDNS (free)    │
                           │  (DNS-01 challenges) │
                           └──────────────────────┘
 ```

docs/index.md

@@ -11,7 +11,7 @@ A reverse proxy and HTTPS termination service using OpenResty/nginx with a manag
 
 - **Automated HTTPS/SSL** - Let's Encrypt integration with HTTP-01 and DNS-01 challenges
 - **Wildcard SSL Certificates** - Support for wildcard domains with automatic renewal
-- **Multiple DNS Providers** - Cloudflare, DigitalOcean, PorkBun integrations
+- **Multiple DNS Providers** - Cloudflare, DigitalOcean, PorkBun, DuckDNS (free) integrations
 - **Advanced Routing** - Sophisticated wildcard domain matching (*, **)
 - **RESTful API** - Full programmatic control
 - **Web Interface** - User-friendly management GUI