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).

go.sum

@@ -2,7 +2,11 @@ github.com/BurntSushi/toml v1.4.0 h1:kuoIxZQy2WRRk1pttg9asf+WVv6tWQuBNVmK8+nqPr0
 github.com/BurntSushi/toml v1.4.0/go.mod h1:ukJfTF/6rtPPRCnwkur4qwRxa8vTRFBF0uk2lLoLwho=
 github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nosvA=
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
+golang.org/x/crypto v0.58.0 h1:COkYLr4k7nDI4r6QtMML2AfzeIswWHAJthLS6K/M/54=
+golang.org/x/crypto v0.58.0/go.mod h1:Xh+kl5A+M0gMiWDytH2Hhr5Z7WbNCpv/eB/EFgfSk+w=
 golang.org/x/image v0.21.0 h1:c5qV36ajHpdj4Qi0GnE0jUc/yuo33OLFaa0d+crTD5s=
 golang.org/x/image v0.21.0/go.mod h1:vUbsLavqK/W303ZroQQVKQ+Af3Yl6Uz1Ppu5J/cLz78=
-golang.org/x/sys v0.4.0 h1:Zr2JFtRQNX3BCZ8YtxRE9hNJYC8J6I1MVbMg6owUp18=
-golang.org/x/sys v0.4.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.49.0 h1:XbzkgYJHdqh/8m2Uu0W/dQv8nktxx4BFHp1M0gROTXA=
+golang.org/x/sys v0.49.0/go.mod h1:hNLxWAXmnKAxqDtdwIYC4bM9oQPEecfsnNMuSxOs3og=
+golang.org/x/time v0.16.0 h1:vMb6ptszcQMkcwiRTAuNNU50gom6++Q/6gY2hDM6VDE=
+golang.org/x/time v0.16.0/go.mod h1:rVKOqvZeKvrDKTQiAHJ7wmwP0RzleSphoEA9RcdLA0s=