Add server-side sessions, session cookie and logout

A successful OIDC callback now creates a sessions row through the new
auth.SessionManager: a random 256-bit ID whose SHA-256 hash is the only
stored form, the subject, email, name and groups claims, a separate random
CSRF token and created/last_seen/expires timestamps. The ID travels only in
the __Host-mealquest_session cookie (HttpOnly, Secure, SameSite=Lax, Path=/,
no Domain).

SessionFor rejects a session unused for MEALQUEST_SESSION_IDLE or older
than MEALQUEST_SESSION_MAX, deletes it, and otherwise slides last_seen_at.
POST /auth/logout deletes the row, clears the cookie and redirects to the
provider's end_session_endpoint, which discovery now requires.

The session lifetimes are optional configuration (defaults 720h and 4320h,
per D-004); set values must be positive with MAX at least IDLE.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/config/config_test.go

@@ -246,6 +246,39 @@ func TestLoad(t *testing.T) {
 			env:     with(map[string]string{EnvOIDCClientID: "  "}),
 			badVars: []string{EnvOIDCClientID},
 		},
+		{
+			name: "session lifetimes set",
+			env:  with(map[string]string{EnvSessionIdle: "24h", EnvSessionMax: "48h"}),
+		},
+		{
+			name: "session max equal to idle",
+			env:  with(map[string]string{EnvSessionIdle: "24h", EnvSessionMax: "24h"}),
+		},
+		{
+			name:    "invalid session idle",
+			env:     with(map[string]string{EnvSessionIdle: "a month"}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "empty session idle",
+			env:     with(map[string]string{EnvSessionIdle: ""}),
+			badVars: []string{EnvSessionIdle},
+		},
+		{
+			name:    "non-positive session max",
+			env:     with(map[string]string{EnvSessionMax: "-1h"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name:    "session max below idle",
+			env:     with(map[string]string{EnvSessionIdle: "48h", EnvSessionMax: "24h"}),
+			badVars: []string{EnvSessionMax},
+		},
+		{
+			name:    "session max below default idle",
+			env:     with(map[string]string{EnvSessionMax: "719h"}),
+			badVars: []string{EnvSessionMax},
+		},
 		{
 			name: "several invalid values are all named",
 			env: with(map[string]string{
@@ -351,6 +384,19 @@ func TestLoadValues(t *testing.T) {
 	if want := []string{"mealquest-admin", "family"}; !slices.Equal(cfg.OIDCAllowedGroups, want) {
 		t.Errorf("OIDCAllowedGroups = %q, want %q", cfg.OIDCAllowedGroups, want)
 	}
+	if cfg.SessionIdle != 720*time.Hour || cfg.SessionMax != 4320*time.Hour {
+		t.Errorf("SessionIdle, SessionMax = %v, %v, want defaults 720h, 4320h", cfg.SessionIdle, cfg.SessionMax)
+	}
+
+	env[EnvSessionIdle] = "12h"
+	env[EnvSessionMax] = "36h"
+	cfg, err = load(func(k string) (string, bool) { v, ok := env[k]; return v, ok })
+	if err != nil {
+		t.Fatalf("load() error = %v", err)
+	}
+	if cfg.SessionIdle != 12*time.Hour || cfg.SessionMax != 36*time.Hour {
+		t.Errorf("SessionIdle, SessionMax = %v, %v, want 12h, 36h", cfg.SessionIdle, cfg.SessionMax)
+	}
 }
 
 func TestSecretRedacted(t *testing.T) {