feat(pipeline): implement FHIR validation pipeline step

[trobanga]

Summary

• Adds a new validation pipeline step that validates FHIR resources against an external FHIR validation service (mii-fhir-validator) via HTTP POST to /validateResource
• Produces per-file OperationOutcome reports (*.validation.ndjson) in jobs/<job-id>/validation/ and fails the step if any resource has error-level or fatal-level issues
• Supports configurable concurrent validation requests (max_concurrent_requests, default: 4), resumption (skips files with existing reports), and transparent step semantics (subsequent steps look past validation to find their input data)

Configuration

services:
  validation:
    url: "http://validator:8080/fhir"
    max_concurrent_requests: 4  # optional, default: 4

pipeline:
  enabled_steps:
    - local_import
    - validation  # can go anywhere after import
    - dimp

New files

• internal/services/validation_client.go — HTTP client for FHIR validation service
• internal/pipeline/validation.go — ExecuteValidationStep with concurrent validation
• tests/unit/validation_client_test.go — 9 client unit tests
• tests/unit/pipeline_validation_test.go — 10 pipeline step unit tests

Modified files

• internal/models/config.go — Added ValidationConfig struct
• internal/services/config.go — Added validation config loading
• internal/services/state.go — Added validation to dirs, unified GetStepInputDir into a single backwards walk
• internal/models/validation.go — Added validation step to service URL checks
• cmd/pipeline.go — Replaced stub with real ExecuteValidationStep call
• Test files updated to include validation URL where needed

Closes #7

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.06965% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.73%. Comparing base (2ac2dc4) to head (22f310e).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/pipeline/validation.go	82.06%	32 Missing and 20 partials ⚠️
internal/services/validation_client.go	94.44%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
+ Coverage   84.68%   84.73%   +0.04%     
==========================================
  Files          46       48       +2     
  Lines        4773     5167     +394     
==========================================
+ Hits         4042     4378     +336     
- Misses        567      603      +36     
- Partials      164      186      +22     

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