feat(server-ng): enforce PAT limits and expose Prometheus metrics

[hubcio]

Enforce max_tokens_per_user at PAT create ingress on both planes and
add a real /metrics endpoint with legacy gauge parity. Binary-plane
PAT rewrite errors used to be dropped silently (the client hung until
timeout); they now return a typed deny. The segments gauge reports the
correct scrape-time count instead of reproducing the legacy drift
bug.

[codecov]

Codecov Report

❌ Patch coverage is 57.46606% with 188 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.81%. Comparing base (5ca0bbb) to head (ce707b2).

Files with missing lines	Patch %	Lines
core/server-ng/src/http.rs	0.00%	65 Missing ⚠️
core/server-ng/src/http/metrics.rs	67.50%	49 Missing and 3 partials ⚠️
core/server-ng/src/dispatch.rs	37.93%	36 Missing ⚠️
core/server-ng/src/http/submit.rs	0.00%	17 Missing ⚠️
core/server-ng/src/bootstrap.rs	14.28%	12 Missing ⚠️
core/server-ng/src/pat.rs	94.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3773       +/-   ##
=============================================
- Coverage     75.53%   32.81%   -42.72%     
  Complexity      969      969               
=============================================
  Files          1317     1316        -1     
  Lines        154895   137087    -17808     
  Branches     128519   110789    -17730     
=============================================
- Hits         117004    44990    -72014     
- Misses        34344    90845    +56501     
+ Partials       3547     1252     -2295     

Components	Coverage Δ
Rust Core	21.93% <57.46%> (-53.58%)	⬇️
Java SDK	62.71% <ø> (ø)
C# SDK	71.14% <ø> (-1.13%)	⬇️
Python SDK	92.27% <ø> (ø)
PHP SDK	84.52% <ø> (ø)
Node SDK	95.31% <ø> (+0.18%)	⬆️
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
core/metadata/src/stm/user.rs	88.28% <100.00%> (+0.33%)	⬆️
core/server-ng/src/http/state.rs	0.00% <ø> (ø)
core/simulator/src/replica.rs	100.00% <100.00%> (ø)
core/server-ng/src/pat.rs	85.33% <94.00%> (+85.33%)	⬆️
core/server-ng/src/bootstrap.rs	21.18% <14.28%> (-0.02%)	⬇️
core/server-ng/src/http/submit.rs	0.00% <0.00%> (ø)
core/server-ng/src/dispatch.rs	39.90% <37.93%> (-0.02%)	⬇️
core/server-ng/src/http/metrics.rs	67.50% <67.50%> (ø)
core/server-ng/src/http.rs	44.38% <0.00%> (-2.96%)	⬇️

... and 723 files with indirect coverage changes

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