fix(backend): embed the IANA time-zone database

The taxpayer's zone decides which tax month an income is filed under, and the
config refuses to boot on a zone it cannot resolve rather than misfiling into
UTC. That correctness was resting on the base image happening to ship
/usr/share/zoneinfo — true of distroless/static-debian12 today, but not a thing
to leave implicit on a path where being wrong means a wrong tax return.

Costs about 450 KB of binary. Adds a config test pinning the default zone and
the hard failure on an unknown one.

backend/internal/config/config_test.go

@@ -202,3 +202,50 @@ func TestLoadGuestReaperDefaultsAndOverride(t *testing.T) {
 		t.Fatal("Load: expected an error for a non-positive reap interval, got nil")
 	}
 }
+
+// TestLoadMyNalogTimeZone pins the taxpayer's time zone: it defaults to Europe/Moscow and an
+// unresolvable zone is a hard boot failure, never a quiet fall back to UTC.
+//
+// The offset a receipt carries decides which tax month an income is filed under, so a silent UTC
+// default would misfile every near-midnight payment at a month boundary. That is also why the
+// backend embeds the IANA database (a blank time/tzdata import in cmd/backend) instead of trusting
+// the base image to ship one.
+func TestLoadMyNalogTimeZone(t *testing.T) {
+	base := func(t *testing.T) {
+		t.Helper()
+		t.Setenv("BACKEND_POSTGRES_DSN", testDSN)
+		t.Setenv("BACKEND_DICT_DIR", "/dict")
+	}
+
+	t.Run("defaults to the taxpayer's zone", func(t *testing.T) {
+		base(t)
+		t.Setenv("BACKEND_MYNALOG_TZ", "")
+		c, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if got := c.MyNalog.Location.String(); got != "Europe/Moscow" {
+			t.Fatalf("time zone = %q, want Europe/Moscow", got)
+		}
+	})
+
+	t.Run("honours an explicit zone", func(t *testing.T) {
+		base(t)
+		t.Setenv("BACKEND_MYNALOG_TZ", "Asia/Yekaterinburg")
+		c, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if got := c.MyNalog.Location.String(); got != "Asia/Yekaterinburg" {
+			t.Fatalf("time zone = %q", got)
+		}
+	})
+
+	t.Run("refuses an unresolvable zone", func(t *testing.T) {
+		base(t)
+		t.Setenv("BACKEND_MYNALOG_TZ", "Mars/Olympus_Mons")
+		if _, err := Load(); err == nil {
+			t.Fatal("an unknown time zone must fail the boot, not fall back to UTC")
+		}
+	})
+}