enrollment: the server mints the §2.1 bootstrap link, the app consumes it

POST /admin/enroll-tokens now returns the whole link, not just the token:

  echolot://enroll?v=1&u=<control URL>&p=pin-sha256:<b64>&t=<token>

The server is the only party that knows all three parts at once, and the part
an operator gets wrong by hand is the base64 pin — which does not fail loudly,
it just never matches, surfacing days later as an inscrutable TLS error. The
app takes the link from a paste or from an echolot:// deep link (QR scan), and
writes URL, pin and credential together or not at all.

One trap the tests pin: an unencoded "+" in a query string decodes to a space,
so a hand-assembled link arrives with a pin wrong by one character. Base64 has
no spaces, so they are restored — unambiguous, and it cannot damage a correctly
encoded pin.

Also fixes a spec divergence: §2.1 names the field device_credential and the
first implementation shipped "credential". Both are sent now and the client
prefers the spec's; the alias goes once nothing reads it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/internal/config/config.go

@@ -63,6 +63,10 @@ type Config struct {
 	MinAppVersion string // ECHOLOT_MIN_APP_VERSION / --min-app-version
 	MaxAppVersion string // ECHOLOT_MAX_APP_VERSION / --max-app-version (exclusive)
 
+	// Where clients reach the control plane, for enrollment links. Empty = derive from the
+	// first control listen address.
+	PublicControlURL string // ECHOLOT_PUBLIC_URL / --public-url
+
 	// Mode
 	Docker bool // --docker (or autodetected; env ECHOLOT_DOCKER=1 forces)
 }
@@ -111,6 +115,7 @@ func Load(args []string) (*Config, *Actions, error) {
 	fs.IntVar(&c.UploadRetentionDays, "upload-retention-days", envInt("UPLOAD_RETENTION_DAYS", 90), "delete uploaded runs older than this; 0 disables")
 	fs.IntVar(&c.UploadMaxRuns, "upload-max-runs", envInt("UPLOAD_MAX_RUNS", 200), "keep at most this many runs per device; 0 disables")
 	fs.StringVar(&c.UploadMinAnon, "upload-min-anonymization", envOr("UPLOAD_MIN_ANONYMIZATION", "full"), "least anonymization accepted: full|balanced|strict")
+	fs.StringVar(&c.PublicControlURL, "public-url", envOr("PUBLIC_URL", ""), "public control-plane URL for enrollment links, e.g. https://probe.example.net:8443")
 	fs.StringVar(&c.MinAppVersion, "min-app-version", envOr("MIN_APP_VERSION", "0.2.0"), "oldest app version this server will serve (SemVer, inclusive)")
 	fs.StringVar(&c.MaxAppVersion, "max-app-version", envOr("MAX_APP_VERSION", "1.0.0"), "first app version this server will refuse (SemVer, exclusive); empty = unbounded")
 	fs.BoolVar(&c.Docker, "docker", envOr("DOCKER", "") == "1", "force container mode (config from env, no systemd/self-update)")