Move upload auth from nginx into the app

Add HTTP Basic auth on /upload, reading an Apache htpasswd file (bcrypt
only, `htpasswd -B`) via a new -htpasswd flag. This lets nginx drop to a
plain TLS-terminating reverse proxy with no auth of its own; the existing
bcrypt htpasswd file is reused unchanged.

auth.go:
- LoadHtpasswd parses user:hash lines, keeps bcrypt entries ($2a/$2b/$2y,
  so Apache's $2y$ works), skips MD5/SHA/crypt with a warning.
- Unknown users still pay for one bcrypt comparison against a dummy hash,
  so response timing can't enumerate valid usernames.
- Failed attempts draw from a token-bucket limiter (x/time/rate, ~1/s,
  burst 10) returning 429 once drained; successful and bulk uploads never
  touch it.

server.go guards both /upload routes (nil authenticator = passthrough for
local --serve). /health stays open. Empty -htpasswd disables auth and logs
a warning.

module.nix: new top-level services.photogallery.htpasswdFile, delivered to
the service via systemd LoadCredential (so a root-owned 0400 secret works).
Removed nginx.htpasswdFile and basic_auth from the vhost.

Both uploader apps already send Basic credentials, so no client change.

Deps: golang.org/x/crypto/bcrypt, golang.org/x/time/rate (vendorHash bump).

server.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/BurntSushi/toml"
+	"golang.org/x/time/rate"
 )
 
 type Server struct {
@@ -19,22 +20,37 @@ type Server struct {
 	outputDir  string
 	serve      bool
 	generator  *Generator
+	auth       *Authenticator // nil disables auth (dev only)
+	authLimit  *rate.Limiter
 }
 
-func NewServer(port, contentDir, outputDir string, serve bool, g *Generator) *Server {
+func NewServer(port, contentDir, outputDir string, serve bool, g *Generator, auth *Authenticator) *Server {
 	return &Server{
 		port:       port,
 		contentDir: contentDir,
 		outputDir:  outputDir,
 		serve:      serve,
 		generator:  g,
+		auth:       auth,
+		// Brute-force throttle: ~1 failed auth/sec steady, burst of 10. Only
+		// failures draw from it, so legitimate bulk uploads are never slowed.
+		authLimit: rate.NewLimiter(rate.Every(time.Second), 10),
 	}
 }
 
+// guard applies HTTP Basic auth to h when an Authenticator is configured, and
+// passes it through untouched otherwise (local --serve development).
+func (s *Server) guard(h http.HandlerFunc) http.HandlerFunc {
+	if s.auth == nil {
+		return h
+	}
+	return s.auth.basicAuth(s.authLimit, h)
+}
+
 func (s *Server) Listen() error {
 	mux := http.NewServeMux()
-	mux.HandleFunc("GET /upload", s.handleUploadPage)
-	mux.HandleFunc("POST /upload", s.handleUpload)
+	mux.HandleFunc("GET /upload", s.guard(s.handleUploadPage))
+	mux.HandleFunc("POST /upload", s.guard(s.handleUpload))
 	mux.HandleFunc("GET /health", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintln(w, "ok")
 	})