chore(release): prepare version 0.12.2

[dipamc]

Prepare the lockstep 0.12.2 release of flopscope, flopscope-server and flopscope-client following the green post-merge CI on #316.

Updates all eight version and cross-package pin locations, refreshes the three lockfiles, and finalizes participant-facing release notes with billing impacts. Third-party dependency versions and the NumPy requirement remain unchanged; uv also normalizes redundant lockfile markers.

Validation:

• All three wheels and source distributions built successfully from these release files.
• Wheel metadata confirms version 0.12.2 and exact cross-package dependency pins.
• Actual-wheel release contract: 6 passed, including isolated Python 3.10 server/client installs and the budget-summary contract.
• All eight version locations agree; all three lockfile checks and diff checks pass.

This PR does not create a tag or trigger PyPI publishing. After CI and merge, tag creation and publishing require a separate explicit user instruction.