Make the HSTS max-age configurable with HSTS_MAX_AGE (closes #271)

HSTS_MAX_AGE, a Go duration, sets the max-age of the
Strict-Transport-Security header. Unset, it is ten years (87600h);
0 sends no header at all. A negative value, or one that is not a
whole number of seconds, stops startup like an unparseable one.
includeSubDomains and preload stay in the header. The value is shown
on /settings and in the startup log, and the README's HSTS section
now says what each part of the header commits the operator to.

Model: opus-5-5

internal/config/config_test.go

@@ -379,6 +379,83 @@ func testSessionIdleTimeoutSuccess(
 	assert.Equal(t, expected, cfg.SessionIdleTimeout)
 }
 
+func TestHSTSMaxAge(t *testing.T) {
+	tests := []struct {
+		name        string
+		set         bool
+		value       string
+		expectError bool
+		// sentinel, when set, must be wrapped by the startup
+		// error; every error case must additionally name the
+		// variable in its message.
+		sentinel error
+		expected time.Duration
+	}{
+		{
+			name:     caseUnsetUsesDefault,
+			set:      false,
+			expected: 87600 * time.Hour,
+		},
+		{
+			name:     caseValidValueParsed,
+			set:      true,
+			value:    "720h",
+			expected: 720 * time.Hour,
+		},
+		{
+			// Zero turns the header off; it is not an error.
+			name:     "zero is accepted",
+			set:      true,
+			value:    "0",
+			expected: 0,
+		},
+		{
+			name:        caseUnparseableFails,
+			set:         true,
+			value:       "10 years",
+			expectError: true,
+		},
+		{
+			// A whole number of seconds, so only the sign is wrong.
+			name:        "negative whole seconds fail startup",
+			set:         true,
+			value:       "-1s",
+			expectError: true,
+			sentinel:    config.ErrInvalidHSTSMaxAge,
+		},
+		{
+			name:        "fraction of a second fails startup",
+			set:         true,
+			value:       "1500ms",
+			expectError: true,
+			sentinel:    config.ErrInvalidHSTSMaxAge,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			configtest.ClearEnv(t)
+			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
+
+			if tt.set {
+				t.Setenv("HSTS_MAX_AGE", tt.value)
+			}
+
+			if tt.expectError {
+				expectStartupErrorFor(t, "HSTS_MAX_AGE", tt.sentinel)
+
+				return
+			}
+
+			cfg, err := buildConfig(t)
+			require.NoError(t, err)
+			assert.Equal(t, tt.expected, cfg.HSTSMaxAge)
+		})
+	}
+}
+
 func TestDefaultDataDir(t *testing.T) {
 	for _, env := range []string{"", "dev", "prod"} {
 		name := env