fix: track multiple hostnames per IP:port in port state (#65)

## Summary

Port state keys are `ip:port` with a single `hostname` field. When multiple hostnames resolve to the same IP (shared hosting, CDN), only one hostname was associated. This caused orphaned port state when that hostname removed the IP from DNS while the IP remained valid for other hostnames.

## Changes

### State (`internal/state/state.go`)
- `PortState.Hostname` (string) → `PortState.Hostnames` ([]string)
- Custom `UnmarshalJSON` for backward compatibility: reads old single `hostname` field and migrates to a single-element `hostnames` slice
- Added `DeletePortState` and `GetAllPortKeys` methods for cleanup

### Watcher (`internal/watcher/watcher.go`)
- Refactored `checkAllPorts` into three phases:
  1. Build IP:port → hostname associations from current DNS data
  2. Check each unique IP:port once with all associated hostnames
  3. Clean up stale port state entries with no hostname references
- Port change notifications now list all associated hostnames (`Hosts:` instead of `Host:`)
- Added `buildPortAssociations`, `parsePortKey`, and `cleanupStalePorts` helper functions

### README
- Updated state file format example: `hostname` → `hostnames` (array)
- Updated notification description to reflect multiple hostnames

## Backward Compatibility

Existing state files with the old single `hostname` string are handled gracefully via custom JSON unmarshaling — they are read as single-element `hostnames` slices.

Closes #55

Co-authored-by: clawbot <clawbot@noreply.eeqj.de>
Reviewed-on: #65
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -110,8 +110,8 @@ includes:
 - **NS recoveries**: Which nameserver recovered, which hostname/domain.
 - **NS inconsistencies**: Which nameservers disagree, what each one
   returned, which hostname affected.
-- **Port changes**: Which IP:port, old state, new state, associated
-  hostname.
+- **Port changes**: Which IP:port, old state, new state, all associated
+  hostnames.
 - **TLS expiry warnings**: Which certificate, days remaining, CN,
   issuer, associated hostname and IP.
 - **TLS certificate changes**: Old and new CN/issuer/SANs, associated
@@ -290,12 +290,12 @@ not as a merged view, to enable inconsistency detection.
   "ports": {
     "93.184.216.34:80": {
       "open": true,
-      "hostname": "www.example.com",
+      "hostnames": ["www.example.com"],
       "lastChecked": "2026-02-19T12:00:00Z"
     },
     "93.184.216.34:443": {
       "open": true,
-      "hostname": "www.example.com",
+      "hostnames": ["www.example.com"],
       "lastChecked": "2026-02-19T12:00:00Z"
     }
   },