Small cleanups left after the strictness refactor #21

Found during #17 review; none blocks anything.

• 3D model counting (utils/metadata.count_model_files) ignores upper-case suffixes such as .STEP, which vendors use. Count case-insensitively.
• utils/file_ops reads quoted lib-table values without undoing the escaping services/library_table writes.
• utils/backup prunes in name order, which is wrong when the clock moves back (DST end); prune by mtime or parse the timestamp.
• utils/template/selection.find_potential_variables ignores its patterns parameter.
• commands/update/__init__ and config/__init__ re-export functions nobody uses.
• list_libraries prints a redundant 'No libraries found'; update/perform.py repeats its dry-run text.
• find_kicad_config ignores KICAD_CONFIG_HOME, XDG_CONFIG_HOME and Flatpak config locations.
• Template extends (inheritance) is still unimplemented; create only warns.
• Startup: load each command module only when it runs (would cut most of the ~140 ms import on every Python, beyond the PEP 810 __lazy_modules__ gain on 3.15).