Add a read-only Settings page for the loaded configuration (closes #402)

The page at /settings, behind the login and linked from the
navigation bar, lists every field of the configuration the server
started with: each by its environment variable name, with the
README's description and the value in effect. METRICS_PASSWORD and
SENTRY_DSN show only as set or not set; their values never reach the
template. The handlers now take the loaded Config from the dependency
graph.

Model: opus-5-5

internal/handlers/settings.go

@@ -0,0 +1,130 @@
+package handlers
+
+import (
+	"net/http"
+	"net/netip"
+	"strconv"
+	"strings"
+
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// notSet is what the Settings page shows for a value that is empty.
+const notSet = "not set"
+
+// settingRow is one line of the Settings page: an environment
+// variable, what it controls, and the value the server loaded for it.
+type settingRow struct {
+	Name        string
+	Description string
+	Value       string
+}
+
+// HandleSettings returns a handler for the read-only Settings page,
+// which lists the configuration the server started with.
+func (h *Handlers) HandleSettings() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		h.renderTemplate(w, r, "settings.html", map[string]any{
+			"Settings": settingRows(h.params.Config),
+		})
+	}
+}
+
+// settingRows lists every field of cfg under the environment variable
+// it is read from, in the order of the README's configuration table.
+// METRICS_PASSWORD and SENTRY_DSN are credentials, so their values
+// never reach the page: only whether they are set.
+func settingRows(cfg *config.Config) []settingRow {
+	metricsUsername := cfg.MetricsUsername
+	if metricsUsername == "" {
+		metricsUsername = notSet
+	}
+
+	return []settingRow{
+		{"WEBHOOKER_ENVIRONMENT", "dev or prod", cfg.Environment},
+		{"PORT", "HTTP listen port", strconv.Itoa(cfg.Port)},
+		{
+			"BIND_ADDRESS",
+			"IP address the HTTP listener binds",
+			cfg.BindAddress,
+		},
+		{
+			"DATA_DIR",
+			"Directory for all SQLite databases",
+			cfg.DataDir,
+		},
+		{
+			"DEBUG",
+			"Enable debug logging",
+			strconv.FormatBool(cfg.Debug),
+		},
+		{
+			"MAINTENANCE_MODE",
+			"Report maintenanceMode: true in the healthcheck JSON. " +
+				"It does not change how any request is served",
+			strconv.FormatBool(cfg.MaintenanceMode),
+		},
+		{
+			"METRICS_USERNAME",
+			"Basic auth username for /metrics",
+			metricsUsername,
+		},
+		{
+			"METRICS_PASSWORD",
+			"Basic auth password for /metrics",
+			setOrNotSet(cfg.MetricsPassword),
+		},
+		{
+			"SENTRY_DSN",
+			"Error reporting DSN. Unset leaves error reporting off",
+			setOrNotSet(cfg.SentryDSN),
+		},
+		{
+			"RETENTION_SWEEP_INTERVAL",
+			"How often the retention reaper and archive sweeper run",
+			cfg.RetentionSweepInterval.String(),
+		},
+		{
+			"SESSION_IDLE_TIMEOUT",
+			"Idle session timeout. Zero or negative disables idle " +
+				"expiry",
+			cfg.SessionIdleTimeout.String(),
+		},
+		{
+			"RECEIVER_RATE_LIMIT",
+			"Receiver requests per minute per IP per entrypoint " +
+				"(10x that per IP across the route)",
+			strconv.Itoa(cfg.ReceiverRateLimit),
+		},
+		{
+			"TRUSTED_PROXIES",
+			"CIDRs whose forwarded headers are trusted",
+			cidrList(cfg.TrustedProxies),
+		},
+		{
+			"ALLOWED_EGRESS_CIDRS",
+			"CIDRs that delivery targets may reach despite the " +
+				"SSRF blocklist",
+			cidrList(cfg.AllowedEgressCIDRs),
+		},
+	}
+}
+
+// setOrNotSet is how the Settings page shows a credential: whether it
+// has a value, never the value itself.
+func setOrNotSet(value string) string {
+	if value == "" {
+		return notSet
+	}
+
+	return "set"
+}
+
+// cidrList renders a CIDR list setting for the Settings page.
+func cidrList(prefixes []netip.Prefix) string {
+	if len(prefixes) == 0 {
+		return "none"
+	}
+
+	return strings.Join(config.PrefixStrings(prefixes), ", ")
+}