upaas: health check, settings checked at start, README section (closes #59)

The image's HEALTHCHECK requests /.well-known/healthcheck through
nginx on the port from PORT, so it fails unless both processes answer.
The backend reads PORT and DEBUG with strconv instead of viper, which
turned a bad PORT into 0 and a bad DEBUG into false. Those, and a
BIND_ADDRESS that is not an IP address, now stop the start with an
error naming the variable; the TRUSTED_PROXIES error names it too.
README.md gains "Running under upaas". Its first-run steps create the
host directory owned by uid 1000, so the image changes no ownership.

Model: opus-5-5

backend/internal/middleware/middleware.go

@@ -77,8 +77,8 @@ func New(
 	return s, nil
 }
 
-// parseTrustedProxies converts CIDR strings into prefixes,
-// failing fast on any malformed entry.
+// parseTrustedProxies converts the TRUSTED_PROXIES entries into
+// prefixes, failing fast on any malformed entry.
 func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 	prefixes := make([]netip.Prefix, 0, len(cidrs))
 
@@ -86,7 +86,7 @@ func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 		prefix, err := netip.ParsePrefix(cidr)
 		if err != nil {
 			return nil, fmt.Errorf(
-				"trusted proxy %q: %w", cidr, err,
+				"TRUSTED_PROXIES %q: %w", cidr, err,
 			)
 		}
 

backend/internal/middleware/middleware_test.go

@@ -40,8 +40,8 @@ func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
 	t.Parallel()
 
 	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
-	if err == nil {
-		t.Fatal("expected error for malformed CIDR, got nil")
+	if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
+		t.Fatalf("error = %v, want one naming TRUSTED_PROXIES", err)
 	}
 }