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/server/routes_test.go

@@ -24,6 +24,7 @@ import (
 	"sneak.berlin/go/webhooker/internal/middleware"
 	"sneak.berlin/go/webhooker/internal/server"
 	"sneak.berlin/go/webhooker/internal/session"
+	"sneak.berlin/go/webhooker/static"
 )
 
 // csrfCookieName is the cookie gorilla/csrf issues when it runs. Its
@@ -246,6 +247,56 @@ func (e *testEnv) storedHash(t *testing.T, username string) string {
 	return user.Password
 }
 
+// --- /s static group ---
+
+// TestStaticServesEveryMethod pins what the static mount actually
+// answers. chi's Mount registers the handler for all methods and
+// http.FileServer only special-cases HEAD (by suppressing the body),
+// so a POST or a DELETE to an asset is served the file rather than
+// refused. The README documents this; the test is what keeps the two
+// from drifting.
+func TestStaticServesEveryMethod(t *testing.T) {
+	t.Parallel()
+
+	env := newTestEnv(t)
+
+	body, err := static.Static.ReadFile("js/app.js")
+	require.NoError(t, err)
+	require.NotEmpty(t, body)
+
+	for _, method := range []string{
+		http.MethodGet,
+		http.MethodHead,
+		http.MethodPost,
+		http.MethodPut,
+		http.MethodDelete,
+	} {
+		t.Run(method, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				context.Background(), method,
+				"/s/js/app.js", nil,
+			)
+			w := httptest.NewRecorder()
+			env.router.ServeHTTP(w, req)
+
+			assert.Equal(t, http.StatusOK, w.Code,
+				"static mount answers every method")
+
+			if method == http.MethodHead {
+				assert.Empty(t, w.Body.Bytes(),
+					"HEAD must not carry a body")
+
+				return
+			}
+
+			assert.Equal(t, body, w.Body.Bytes(),
+				"the asset itself is returned")
+		})
+	}
+}
+
 // --- /pages group ---
 
 // TestPagesLogin_OversizeBody_RejectedBeforeCSRF proves the cap runs