fix(security): close High/Critical findings from the Stage 8 scan

- Bump github.com/rabbitmq/amqp091-go 1.12.0 -> 1.13.0 (3 Critical, 7 High,
  reachable via rabbitmq.Connect) and golang.org/x/{text,net,crypto},
  klauspost/compress; toolchain go1.26.8, builder image pinned to
  golang:1.26.8-alpine3.24.
- Build Content-Disposition per RFC 6266/8187 (ASCII fallback +
  filename*=UTF-8''), stripping CR/LF, quotes and backslashes (CWE-113).
- Cap request bodies with MaxBytesReader (HTTP_MAX_UPLOAD_BYTES, default
  2 GiB); multipart uploads over the limit return 413 (CWE-400).
- /readyz reports "unavailable" instead of raw backend errors (CWE-209).
- Drop deprecated middleware.RealIP; add X-Content-Type-Options: nosniff.
- docker-compose credentials come from ${VAR:-default}.
- Check the RegisterValidation error; annotate verified gosec false
  positives with #nosec and a reason.

Re-scan: govulncheck 0 reachable, trivy fs/image 0 High/Critical,
gosec High 0. go test -race passes against PostGIS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/config/config.go

@@ -27,6 +27,9 @@ type HTTPConfig struct {
 	WriteTimeout      time.Duration `env:"HTTP_WRITE_TIMEOUT" envDefault:"120s"`
 	IdleTimeout       time.Duration `env:"HTTP_IDLE_TIMEOUT" envDefault:"60s"`
 	ShutdownTimeout   time.Duration `env:"HTTP_SHUTDOWN_TIMEOUT" envDefault:"10s"`
+	// MaxUploadBytes caps the size of any request body, multipart uploads
+	// included (default 2 GiB); larger requests get 413. 0 disables the cap.
+	MaxUploadBytes int64 `env:"HTTP_MAX_UPLOAD_BYTES" envDefault:"2147483648"`
 }
 
 // Addr returns the listen address for the HTTP server.