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