feat(server): Add jemalloc stats as Datadog metrics

[jan-auer]

Objectstore shows RSS growth over time in production. Before investing in heap profiling, we need to distinguish real allocation growth from allocator fragmentation. This adds four jemalloc internal stats as Datadog gauges emitted on the existing metrics_interval (default 10s): jemalloc.allocated, jemalloc.active, jemalloc.resident, and jemalloc.mapped. Comparing allocated (what the app requested) against resident (what the OS reports) will make that distinction visible in production.

Also bumps tikv-jemallocator from 0.6.1 to 0.7.0 (jemalloc 5.3.1, no breaking changes) and adds tikv-jemalloc-ctl 0.7.0 with the stats feature. Both must be the same version to link against the same tikv-jemalloc-sys.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.39%. Comparing base (cfee5af) to head (106008b).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
objectstore-server/src/state.rs	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
+ Coverage   87.37%   87.39%   +0.02%     
==========================================
  Files          86       86              
  Lines       13761    13825      +64     
==========================================
+ Hits        12023    12082      +59     
- Misses       1738     1743       +5     

Components	Coverage Δ
Rust Backend	92.19% <96.29%> (-0.01%)	⬇️
Rust Client	80.06% <ø> (+0.35%)	⬆️
Python Client	88.51% <ø> (ø)

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