Add support for push idempotency keys

[varmar05]

Cache responses for push finish actions so we can return them to client as they were in case of repeated calls (e.g. on network issues).

[coveralls]

Coverage Report for CI Build 26095869811

Coverage increased (+0.08%) to 93.299%

Details

• Coverage increased (+0.08%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (172 of 174 lines covered, 98.85%).
• 28 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
server/mergin/sync/models.py	13	11	84.62%

Coverage Regressions

28 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
server/mergin/sync/storages/disk.py	26	82.47%
server/mergin/tests/test_disk_utils.py	2	96.23%

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Coverage Stats

Relevant Lines:	9983
Covered Lines:	9314
Line Coverage:	93.3%
Coverage Strength:	0.93 hits per line

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💛 - Coveralls