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).

flake.nix

@@ -18,7 +18,7 @@
 
           # Run `go mod tidy` to generate go.sum, then `nix build`.
           # Nix will fail and print the correct hash — paste it here.
-          vendorHash = "sha256-SxQDRProQtauWXPvPzbZZChLQd0iey9hnXFLiPYTS98=";
+          vendorHash = "sha256-Q+VeDZX9DlZtOZrYRwZo19eyob8jIu19IRhdvChmVL4=";
 
           meta = with pkgs.lib; {
             description = "Minimal static photo gallery";