Add per-agent log retention floor, owner-only to set or override

api/agents.ConfigOverride gains LogRetentionDays: a per-agent setting
edited on the same remote-config page as extra_file_paths, but unlike
every other field there it's central-policy metadata api/logretention
reads, never something the agent process itself sees. Any change to
it -- setting, raising, lowering, or clearing -- requires RoleOwner,
not just RoleAdmin: the whole point of the field is a floor an admin
can't move, so an admin able to freely edit it would defeat that.

api/logretention now checks the largest LogRetentionDays configured
across any agent (AgentRetentionStore, new) before every preview/delete:
a non-owner's request is rejected with a clear 403 if it would reach
into that protected window. An owner always bypasses it, matching "make
the log retention override any attempts to delete logs by anyone other
than owner role."

Verified live end-to-end: owner sets a 90-day floor on an agent, admin
is blocked deleting anything newer than that (both preview and delete),
allowed beyond it, and owner bypasses it entirely -- confirmed against
real ClickHouse data, not just the fake-backed unit tests. Also caught
and fixed a real pre-existing latent bug while verifying in-browser: a
type="number" Input's bind:value becomes an actual JS number once a
user types into it (only the initial value is a string), which broke a
bare .trim() call on the new field.

api/agents/handler_test.go

@@ -309,6 +309,73 @@ func TestHandleSetConfigExtraFilePathsRequiresAdminToAdd(t *testing.T) {
 	}
 }
 
+// TestHandleSetConfigLogRetentionDaysRequiresOwner is the analogous
+// regression test for log_retention_days -- but unlike extra_file_paths,
+// there is no safe direction an Admin is allowed to move it in: setting,
+// raising, lowering, and clearing all require Owner (see
+// changesLogRetentionDays's doc comment for why).
+func TestHandleSetConfigLogRetentionDaysRequiresOwner(t *testing.T) {
+	s := newFakeStore()
+	s.put(Agent{TenantID: "default", Host: "web-01"})
+	editor := NewHandler(discardLogger(), s, fakeAuthorizer{role: authz.RoleEditor}, nil)
+	admin := NewHandler(discardLogger(), s, fakeAuthorizer{role: authz.RoleAdmin}, nil)
+	owner := NewHandler(discardLogger(), s, fakeAuthorizer{role: authz.RoleOwner}, nil)
+
+	days90 := 90
+	rec := doRequest(t, admin, "PUT", "/agents/web-01/config", ConfigOverride{LogRetentionDays: &days90})
+	if rec.Code != http.StatusForbidden {
+		t.Fatalf("admin setting log_retention_days: status = %d, want 403", rec.Code)
+	}
+
+	rec = doRequest(t, owner, "PUT", "/agents/web-01/config", ConfigOverride{LogRetentionDays: &days90})
+	if rec.Code != http.StatusOK {
+		t.Fatalf("owner setting log_retention_days: status = %d, want 200, body=%s", rec.Code, rec.Body.String())
+	}
+	var got Agent
+	if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decoding response: %v", err)
+	}
+	if got.DesiredOverride == nil || got.DesiredOverride.LogRetentionDays == nil || *got.DesiredOverride.LogRetentionDays != 90 {
+		t.Fatalf("stored override = %+v, want log_retention_days=90", got.DesiredOverride)
+	}
+
+	// Lowering an existing value is exactly as gated as raising it.
+	days30 := 30
+	rec = doRequest(t, admin, "PUT", "/agents/web-01/config", ConfigOverride{LogRetentionDays: &days30})
+	if rec.Code != http.StatusForbidden {
+		t.Fatalf("admin lowering log_retention_days: status = %d, want 403", rec.Code)
+	}
+
+	// Clearing it (omitting the field entirely) is also gated -- an
+	// admin resending the rest of the override without this field must
+	// not silently drop an owner-set floor.
+	rec = doRequest(t, admin, "PUT", "/agents/web-01/config", ConfigOverride{})
+	if rec.Code != http.StatusForbidden {
+		t.Fatalf("admin clearing log_retention_days: status = %d, want 403", rec.Code)
+	}
+
+	// An editor is blocked the same way an admin is -- this floor is
+	// Owner-only, not Admin-or-above like extra_file_paths.
+	rec = doRequest(t, editor, "PUT", "/agents/web-01/config", ConfigOverride{LogRetentionDays: &days30})
+	if rec.Code != http.StatusForbidden {
+		t.Fatalf("editor setting log_retention_days: status = %d, want 403", rec.Code)
+	}
+}
+
+func TestHandleSetConfigRejectsInvalidLogRetentionDays(t *testing.T) {
+	s := newFakeStore()
+	s.put(Agent{TenantID: "default", Host: "web-01"})
+	owner := NewHandler(discardLogger(), s, fakeAuthorizer{role: authz.RoleOwner}, nil)
+
+	for _, days := range []int{0, -1, 3651} {
+		d := days
+		rec := doRequest(t, owner, "PUT", "/agents/web-01/config", ConfigOverride{LogRetentionDays: &d})
+		if rec.Code != http.StatusBadRequest {
+			t.Errorf("log_retention_days=%d: status = %d, want 400", days, rec.Code)
+		}
+	}
+}
+
 func TestHandleSetConfigUnknownHostIsNotFound(t *testing.T) {
 	h := newTestHandler(newFakeStore())
 	interval := int64(30000)