feat: Structured Request Logging (F11)

[leocamello]

Summary

Implements F11: Structured Request Logging — per-request structured logs with correlation IDs, privacy-safe content handling, and component-level filtering.

What's New

Logging Module (src/logging/)

• mod.rs — Module exports + build_filter_directives() for EnvFilter construction
• middleware.rs — UUID v4 request ID generation
• fields.rs — Token extraction, status extraction, privacy-safe prompt truncation

Instrumented Request Handler

• #[instrument] macro on handle_chat_completion() with 15 span fields
• Fields recorded at lifecycle points: request entry → after routing → at completion
• Log level progression: INFO (success) → WARN (retry) → ERROR (exhausted)

Route Reason Tracking

• RoutingResult gains route_reason: String field
• Descriptive reasons: highest_score:backend:0.95, round_robin:index_N, fallback:model:reason

Configuration

• component_levels — per-component log level overrides
• enable_content_logging — opt-in prompt preview (default: off, with startup warning)
• JSON output format via tracing-subscriber JSON layer

Verification Summary

• 437 tests pass (30 new: 20 integration + 7 unit + 3 doc tests)
• Clippy clean, fmt clean
• speckit.analyze: Full alignment (15/15 FR, 6/6 US, 10/10 SC)
• verification.md: 123✅, 71 N/A
• tasks.md: 90/90 tasks complete

Spec Artifacts

• specs/011-structured-logging/spec.md — 15 functional requirements, 6 user stories
• specs/011-structured-logging/walkthrough.md — Junior-developer-friendly code guide

Closes #117
Closes #118
Closes #119
Closes #120
Closes #121
Closes #122
Closes #123
Closes #124

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 73.14286% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.04%. Comparing base (b3e1730) to head (ae2b0a8).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/routing/mod.rs	65.43%	28 Missing ⚠️
src/logging/fields.rs	66.66%	12 Missing ⚠️
src/cli/serve.rs	0.00%	4 Missing ⚠️
src/api/completions.rs	93.47%	3 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
- Coverage   79.96%   79.04%   -0.92%     
==========================================
  Files          37       40       +3     
  Lines        1612     1756     +144     
==========================================
+ Hits         1289     1388      +99     
- Misses        323      368      +45     

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