- decode.py: use the GSMTAP_FLAG_BAD_FCS constant instead of a literal
0x01 in gsmtap_flag_names() (import from ..gsmtap); no behavior change.
- tests: add a DB round-trip test (insert_message with BAD_FCS flag →
get_messages → decoded.errors) covering the end-to-end path, not just
the direct decode_sniff_msg call.