feat(gc): add gcTimeWindow to limit periodic GC to a local time range

[AkashKumar7902]

Summary

• Adds a new optional gcTimeWindow storage setting that confines scheduled garbage collection to a local time range (e.g. 01.00 - 08.00), supporting both . and : separators and windows that cross midnight.
• Honours the window for the default store and each subpath, validates the value at config load, and gates the GC task generator's readiness on the window.
• Updates examples/README.md and examples/config-gc-periodic.json so operators can discover and copy the new option.

Closes #3001

Implementation notes

• pkg/storage/gc/time_window.go exposes ParseTimeWindow / TimeWindow.Contains. The window is empty by default (matches today's behaviour).
• pkg/api/controller.go plumbs the parsed window through gc.Options for both the default store and subpaths; an invalid value logs and skips scheduled GC for that store rather than failing the whole server.
• pkg/cli/server/root.go rejects malformed values up-front and warns when a window is set without gc: true.
• UpdateReloadableConfig carries the new field across reloads so live config edits take effect without a restart.

Test plan

• go test ./pkg/storage/gc/... ./pkg/api/config/... ./pkg/cli/server/...
• golangci-lint run on the touched packages
• Verified parsing of same-day window, midnight-crossing window, and invalid inputs via unit tests
• Verified GC task generator readiness honours the window via unit test

[codecov]

Codecov Report

❌ Patch coverage is 75.78947% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.39%. Comparing base (8282aef) to head (c909154).
⚠️ Report is 52 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/storage/gc/time_window.go	80.39%	6 Missing and 4 partials ⚠️
pkg/api/controller.go	60.00%	5 Missing and 3 partials ⚠️
pkg/cli/server/root.go	64.28%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4017      +/-   ##
==========================================
- Coverage   91.42%   91.39%   -0.04%     
==========================================
  Files         197      198       +1     
  Lines       28152    28237      +85     
==========================================
+ Hits        25739    25806      +67     
- Misses       1562     1572      +10     
- Partials      851      859       +8     

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[rchincha]

@AkashKumar7902 pls fix merge conflicts and ci failures