Fix: narrow zstd compression level to the conventional -22..22 range

The raw library bounds (-131072, 22) come from an internal zstd
implementation constant (-ZSTD_TARGETLENGTH_MAX), not a meaningful
distinct level — deeper negative values than -22 buy nothing over -22
in practice, and the zstd CLI/community convention only uses -22..22.
Exposing the raw range via --zip-compression-level would let a user
pass a number like -50000 that "validates" but means nothing.

src/documents/tests/export/test_compression.py

@@ -78,6 +78,8 @@ class TestLevelError:
             ("deflated", 9),
             ("bzip2", 1),
             ("bzip2", 9),
+            ("zstd", -22),
+            ("zstd", 22),
             ("deflated", None),
             ("stored", None),
         ],
@@ -100,6 +102,8 @@ class TestLevelError:
             ("deflated", -1),
             ("bzip2", 0),
             ("bzip2", 10),
+            ("zstd", -23),
+            ("zstd", 23),
         ],
     )
     def test_out_of_range_levels_return_message(