feat: translation-quality API + 3-way capability merge

[Souptik-De]

Summary

Adds translation-quality ladder (EXACT → LANG → FALLBACK_EN → NONE) to frontend translation API.

Changes

• New public enum in cpdb.h
• New *WithQuality() API for option/choice/group translations (cpdb-frontend.h)
• 3-way capability merge: locale → base language → English with per-field quality tags
• libcpdb_frontend_la version bump 4:0:1 (new API)

Testing

• make check passes
• Integration verified against cpdb-backend-cups
• Builds cleanly on upstream master with multi-arch CI

Notes

• Rebased onto upstream master (db3855b) with multi-arch CI
• Upstream's capability structures (PR Define structures and methods for all printer capabilities and implement functions to create and delete printer capabilities #95) and leak fixes (PR Fix resource leaks on error paths in cpdb-frontend.c #98) integrated
• libcpdb_frontend_la version bumped to 4:0:1 for new API

[tillkamppeter]

libcpdb_frontend_la version bumped to 4:0:1 for new API

Note that a version bump is only needed if the new API is breaking any program made for the previous API. If you only add functions and data structures but you do not remove anything existing or incompatibly modify something existing, you do not need to bump.