Correct release-blocking README and startup-warning inaccuracies (closes #151)

Publishing this README would have shipped false statements about the
product. Corrects the eight items on the issue plus everything a full
sweep turned up: the Slack circuit-breaker scope, a nonexistent WAL, the
wrong config key for slack targets, six undocumented routes, the
conditional /metrics registration, wrong retention bands, wrong shutdown
mechanism, and a Quick Start that led a new contributor into a red
build.

The lockout warning now fires whenever TRUSTED_PROXIES is empty rather
than only in production, since the variable it was gated on defaults to
dev. Rate-limit keying, the limits and the TRUSTED_PROXIES default are
untouched — those belong to #150.

What /s/* actually serves was settled empirically rather than by
reading: all five of GET/HEAD/POST/PUT/DELETE return 200, pinned by
TestStaticServesEveryMethod. Restricting it is filed separately.

Independently reviewed after three prior rounds. The reviewer
re-derived all fifteen claim-table rows against the code, including
every row a previous revision had marked "correct, left alone" and got
wrong, and found zero false; then verified every route method-by-method,
all twelve environment variables, all nine entity tables, and the
package tree against git ls-files. The Quick Start was confirmed by
running it in a fresh clone.

internal/config/config.go

@@ -422,33 +422,43 @@ func loadFromEnv() (*Config, error) {
 	}, nil
 }
 
-// warnSharedRateLimitBucket logs a startup warning when a production
-// deployment leaves TRUSTED_PROXIES empty.
+// warnSharedRateLimitBucket logs a startup warning whenever
+// TRUSTED_PROXIES is empty, in any environment.
 //
 // With no trusted proxies every rate limiter keys on the connecting
-// peer's address. A production deployment is required to run behind a
-// TLS-terminating reverse proxy, and the peer is then that proxy for
-// every request, so all clients share one bucket per limiter. The
+// peer's address. Whether that is harmless or dangerous depends on
+// what is in front of the process, which this code cannot observe:
+// with nothing in front, the peer is the client and the limits are
+// per-client as intended; behind a reverse proxy the peer is the proxy
+// for every request, so all clients share one bucket per limiter. The
 // login limiter's bucket is the dangerous one: any remote client can
-// keep it full, which denies the only administrative login to
-// everyone until the process restarts.
+// keep it full, which denies the only administrative login to everyone
+// until the process restarts.
+//
+// The warning is deliberately not gated on WEBHOOKER_ENVIRONMENT. That
+// variable defaults to dev, so gating on it would silence the warning
+// for exactly the operator who forgot to configure the deployment —
+// the case it exists to catch.
 //
 // The default of trusting nobody is deliberate — trusting forwarded
 // headers from arbitrary peers lets any client choose its own bucket —
 // so this warns rather than failing startup or changing the key.
 func (c *Config) warnSharedRateLimitBucket(log *slog.Logger) {
-	if !c.IsProd() || len(c.TrustedProxies) > 0 {
+	if len(c.TrustedProxies) > 0 {
 		return
 	}
 
 	log.Warn(
-		"TRUSTED_PROXIES is empty: rate limits key on the "+
-			"connecting peer, so behind the reverse proxy a "+
-			"production deployment runs behind, every client "+
-			"shares one bucket per limit. Any remote client can "+
-			"then keep the login limit full and deny the admin "+
-			"login, the only administrative path, until restart. "+
-			"Set TRUSTED_PROXIES to your reverse proxy's address.",
+		"TRUSTED_PROXIES is empty: every rate limit keys on the "+
+			"connecting peer's address. With nothing proxying to "+
+			"this process that is the client itself and the limits "+
+			"are per-client as intended. Behind a reverse proxy the "+
+			"peer is the proxy on every request, so all clients "+
+			"share one bucket per limit and any remote client can "+
+			"keep the login limit full, denying the admin login — "+
+			"the only administrative path — until restart. If "+
+			"anything proxies to this process, set TRUSTED_PROXIES "+
+			"to its address.",
 		"environment", c.Environment,
 		"trustedProxies", len(c.TrustedProxies),
 	)
@@ -491,6 +501,10 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"maintenanceMode", s.MaintenanceMode,
 		"dataDir", s.DataDir,
 		"retentionSweepInterval", s.RetentionSweepInterval.String(),
+		// Logged because a perfectly valid non-positive value here
+		// disables idle expiry entirely, and that is worth showing
+		// back to the operator.
+		"sessionIdleTimeout", s.SessionIdleTimeout.String(),
 		"receiverRateLimit", s.ReceiverRateLimit,
 		"trustedProxies", len(s.TrustedProxies),
 		"hasSentryDSN", s.SentryDSN != "",