feat: implement B.1 — persist login sessions in SQLite with sliding idle timeout

Sessions move from an in-memory map (absolute 12h TTL) to a `sessions`
table (migration 0002), storing only the SHA-256 of the token. Expiry is
now a sliding idle window (PANEL_SESSION_IDLE_DAYS, default 7, no
absolute cap), extended at most once an hour and never by the
monitoring screens' background polling (GET + HX-Request), so a
forgotten open tab doesn't keep a session alive indefinitely. A login
now survives a container restart or redeploy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/session_test.go

@@ -1,9 +1,25 @@
 package web
 
-import "testing"
+import (
+	"path/filepath"
+	"testing"
+	"time"
+
+	"codeberg.org/mix/selfpost/internal/store"
+)
+
+func newTestSessionStore(t *testing.T) *sessionStore {
+	t.Helper()
+	st, err := store.Open(filepath.Join(t.TempDir(), "test.db"))
+	if err != nil {
+		t.Fatalf("open store: %v", err)
+	}
+	t.Cleanup(func() { st.Close() })
+	return newSessionStore(st, 7*24*time.Hour)
+}
 
 func TestSessionRename(t *testing.T) {
-	s := newSessionStore()
+	s := newTestSessionStore(t)
 	token := s.Create("admin")
 
 	s.Rename(token, "operator")
@@ -21,7 +37,7 @@ func TestSessionRename(t *testing.T) {
 // under the old password stops working) while keeping the one performing the
 // change signed in.
 func TestSessionDestroyOthers(t *testing.T) {
-	s := newSessionStore()
+	s := newTestSessionStore(t)
 	keep := s.Create("admin")
 	other := s.Create("admin")
 
@@ -34,3 +50,35 @@ func TestSessionDestroyOthers(t *testing.T) {
 		t.Fatal("other session survived")
 	}
 }
+
+// A session past its sliding idle expiry must not be honoured.
+func TestSessionLookupRejectsExpired(t *testing.T) {
+	s := newTestSessionStore(t)
+	s.idle = -time.Minute // already expired the instant it's created
+	token := s.Create("admin")
+
+	if _, ok := s.Lookup(token); ok {
+		t.Fatal("expired session was accepted")
+	}
+}
+
+// Touch must not rewrite the expiry (or report a renewal) inside the
+// once-an-hour throttle window, so an active tab's polling doesn't turn into
+// a database write per request.
+func TestSessionTouchThrottled(t *testing.T) {
+	s := newTestSessionStore(t)
+	token := s.Create("admin")
+
+	if s.Touch(token) {
+		t.Fatal("touch renewed a session created moments ago")
+	}
+
+	// Back-date the session's last renewal by rewriting its expiry, as if it
+	// had been created (or last renewed) 2 hours ago rather than moments ago.
+	if err := s.store.RenewSession(hashToken(token), time.Now().Add(-2*time.Hour).Add(s.idle)); err != nil {
+		t.Fatalf("renew session: %v", err)
+	}
+	if !s.Touch(token) {
+		t.Fatal("touch did not renew a session past the throttle window")
+	}
+}