fix(flatkv): correct catchup metrics

[blindchaser]

Describe your changes and provide context

• avoid recording CurrentVersion from catchup so read-only historical loads do not pollute the write-mode height metric. Reset PendingWrites after each replayed WAL entry to mirror Commit behavior.

Testing performed to validate your change

[cursor]

PR Summary

Low Risk
Metrics-only changes in the WAL catchup path; no persistence or replay logic is modified, but observability values will change during catchup/replay.

Overview
Stops catchup from recording the CurrentVersion gauge so read-only historical loads don’t pollute the write-mode height metric.

After each replayed WAL entry, catchup now resets per-DB PendingWrites metrics to 0, mirroring the metric behavior of normal Commit after pending buffers are cleared.

^{Reviewed by Cursor Bugbot for commit 51f31aa. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 15, 2026, 10:53 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.29%. Comparing base (823a78d) to head (51f31aa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3447   +/-   ##
=======================================
  Coverage   59.29%   59.29%           
=======================================
  Files        2125     2125           
  Lines      175629   175632    +3     
=======================================
+ Hits       104144   104149    +5     
+ Misses      62404    62402    -2     
  Partials     9081     9081           

Flag	Coverage Δ
sei-chain-pr	75.32% <100.00%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/state_db/sc/flatkv/store_catchup.go	65.41% <100.00%> (+0.79%)	⬆️

... and 1 file with indirect coverage changes

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