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

The empty-TRUSTED_PROXIES warning was gated on IsProd(), but
WEBHOOKER_ENVIRONMENT defaults to dev, so an internet-exposed
deployment whose operator never set it got no warning at all — the
exact operator error the warning exists to catch. It now fires whenever
the list is empty, in any environment, and its text is accurate both
behind a reverse proxy (shared buckets, remotely deniable admin login)
and with nothing in front of the process (harmless). The startup
configuration summary also now logs sessionIdleTimeout, the one value
where a valid setting silently disables a security control.

The README documented a two-stage Docker build on golang:1.24 running
"make check" (the tree has three stages: a golangci-lint lint stage
running fmt-check and lint, a golang:1.26.1-bookworm builder running
test and build, then the Alpine runtime), advertised the public
receiver as accepting all methods (it answers 405 to everything but
POST), claimed unqualified per-IP login rate limiting, and left the
session-expiry prose orphaned inside the trusted-proxy subsection.

The rest of the README was swept against the code rather than only the
reported lines: every documented route checked method-by-method against
internal/server/routes.go (adding the password-change, entrypoint and
target routes that were missing), every environment variable checked
against internal/config/config.go (MAINTENANCE_MODE serves no
maintenance page — it only sets a healthcheck field), the fx wiring,
package tree, prerequisites and dev commands brought back in line with
the tree, and two statements known false from other reviews corrected:
the body-size limit does not reject before "any other middleware" (the
eight global ones run first), and a retention value at or above the
retain-forever sentinel is accepted rather than 400ed.

TODO.md drops the unsupported half of its CI claim, keeping the
cache-defeated container runs, and splits the landed password change
away from the unimplemented reset flow.

internal/config/config_test.go

@@ -628,10 +628,12 @@ func testTrustedProxiesSuccess(
 }
 
 // TestSharedRateLimitBucketWarning covers the startup warning that
-// tells an operator their production deployment shares one rate-limit
-// bucket between every client, which makes the admin login remotely
-// deniable. It must fire when TRUSTED_PROXIES is empty in production
-// and stay quiet otherwise.
+// tells an operator a deployment behind a reverse proxy shares one
+// rate-limit bucket between every client, which makes the admin login
+// remotely deniable. It must fire whenever TRUSTED_PROXIES is empty,
+// in any environment: WEBHOOKER_ENVIRONMENT defaults to dev, so gating
+// on it would silence the warning for exactly the operator who never
+// configured the deployment. It stays quiet once proxies are named.
 func TestSharedRateLimitBucketWarning(t *testing.T) {
 	tests := []struct {
 		name           string
@@ -651,12 +653,19 @@ func TestSharedRateLimitBucketWarning(t *testing.T) {
 			expectWarning:  false,
 		},
 		{
-			// Development is not required to run behind a
-			// reverse proxy, so the shared bucket the warning
-			// describes is not the expected shape there.
-			name:          "dev without trusted proxies is quiet",
+			// The default environment. An internet-exposed
+			// deployment whose operator never set
+			// WEBHOOKER_ENVIRONMENT lands here and has exactly
+			// the exposure the warning announces.
+			name:          "dev without trusted proxies warns",
 			environment:   config.EnvironmentDev,
-			expectWarning: false,
+			expectWarning: true,
+		},
+		{
+			name:           "dev with trusted proxies is quiet",
+			environment:    config.EnvironmentDev,
+			trustedProxies: cidrPrivateV4,
+			expectWarning:  false,
 		},
 	}
 
@@ -697,8 +706,14 @@ func TestSharedRateLimitBucketWarning(t *testing.T) {
 
 			assert.Contains(t, logged, `"level":"WARN"`)
 			assert.Contains(t, logged, "TRUSTED_PROXIES")
-			assert.Contains(t, logged, "shares one bucket")
-			assert.Contains(t, logged, "deny the admin login")
+			assert.Contains(t, logged, "share one bucket")
+			assert.Contains(t, logged, "denying the admin login")
+			// The text must stay accurate for a developer with
+			// nothing in front of the process, where an empty
+			// list costs nothing.
+			assert.Contains(
+				t, logged, "nothing proxying to this process",
+			)
 		})
 	}
 }