feat(prover): change block signing to use timestamp as key

[cyberhorsey]

key previously used block ID which made it difficult to iterate using the DB iterator, because key prefixes would be like blockid-1, blockid-100 instead of something like blockid-1, blockid-2.

also fixes issue where all block hashes were using latestBlock, we now store the block hash as part of the value.

adds a more comprehensive test as well.

[codecov]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (51fd8f9) 60.53% compared to head (8c5286e) 60.56%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
prover/server/api.go	41.93%	17 Missing and 1 partial ⚠️
prover/prover.go	40.00%	9 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #466      +/-   ##
==========================================
+ Coverage   60.53%   60.56%   +0.02%     
==========================================
  Files          45       45              
  Lines        5440     5472      +32     
==========================================
+ Hits         3293     3314      +21     
- Misses       1813     1826      +13     
+ Partials      334      332       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.