fix(auth): invalidate session server-side on logout, not just the cookie

/auth/logout previously only cleared the client's cookie, leaving the
session token valid in the sessions table — a stolen cookie captured
before logout would still work afterwards. Add BackendDbInterface#deleteSession
(implemented in SqliteInterface) and call it from the logout route using
the session token from the cookie. Caught by the new auth-http.test.ts
integration test, updated to assert the session is actually invalidated.

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

test/backends/api/auth-http.test.ts

@@ -74,7 +74,7 @@ describe("api backend: auth over HTTP", async () => {
     assert.equal(res.status, 401);
   });
 
-  it("POST /auth/logout clears the session cookie", async () => {
+  it("POST /auth/logout clears the cookie and invalidates the session server-side", async () => {
     const login = await fetch(`${server.url}/auth/login`, {
       method: "POST",
       headers: { "Content-Type": "application/json" },
@@ -88,6 +88,11 @@ describe("api backend: auth over HTTP", async () => {
     });
     assert.equal(logout.status, 204);
     assert.equal(getCookie(logout, "abode_session"), "");
+
+    const self = await fetch(`${server.url}/auth/self`, {
+      headers: { Cookie: `abode_session=${cookie}` },
+    });
+    assert.equal(self.status, 401, "session was invalidated server-side, not just the cookie cleared");
   });
 
   it("POST /auth/clear-sessions invalidates outstanding session cookies", async () => {