cache sync: log per-archive messages at debug level, fixes #9659

[ThomasWaldmann]

Fixes #9659.

The chunks cache sync emitted 2 INFO lines per archive, so -v / --info floods on repos with many archives:

• warm chunks.archive.d (the usual case): Reading cached archive chunk index for <name> ... + Merging into master chunks index ...
• cache rebuild: Fetching and building archive index for <name> ... + Merging into master chunks index ...

The issue listed three lines (872/875/879), but missed cache.py:804 (Reading cached archive chunk index), which is the partner of the Merging line in the common warm-cache path — without it, only half the noise would go away. This PR demotes all four.

--info output is now just the summary the reporter asked for:

Synchronizing chunks cache...
Archives: %d, w/ cached Idx: %d, w/ outdated Idx: %d, w/o cached Idx: %d.
Done.

Notes:

• Per-archive feedback for interactive use is unaffected: --progress still drives the cache.sync ProgressIndicatorPercent, which names each archive as it is processed.
• borg 2 (master) already logs this whole area at debug level — there is no per-archive INFO logging left in its cache.py — so this aligns the branches.
• ArchiverCorruptionTestCase::test_chunks_archive greps for two of these messages, so its create -v becomes create --debug. (Its third assert is a logger.error and is unaffected.)
• No CHANGES entry, matching the recent 1.4-maint PRs.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.10%. Comparing base (92c7f07) to head (bd8f53e).
⚠️ Report is 1 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/cache.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10217      +/-   ##
=============================================
- Coverage      82.14%   82.10%   -0.04%     
=============================================
  Files             38       38              
  Lines          11475    11475              
  Branches        1807     1807              
=============================================
- Hits            9426     9422       -4     
- Misses          1464     1466       +2     
- Partials         585      587       +2     

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