Add structured standards export: FHIR DiagnosticReport + DICOM SR

[Liohtml]

Part of #51.

Summary

Add additional report output formats alongside the existing PDF/HTML/JSON:
a FHIR DiagnosticReport (JSON) and an optional DICOM Structured Report
(SR, TID 1500). These make MedCheck's output consumable by EHRs, PACS, and
research data platforms — without touching the human-readable reports we already
generate.

Why (vision)

Right now the structured output is a bespoke JSON schema. Emitting
standards-compliant artifacts means another system (a hospital EHR, an imaging
data platform, a downstream validator) can ingest MedCheck results directly, and
it positions findings within recognized terminologies. This is the
"machine-readable, interoperable" companion to the human PDF — purely additive.

Recommended components (license-verified)

• FHIR spec — CC0 (public domain). Python: fhir.resources (BSD,
  Pydantic v2, R4/R5) — clean fit. Emit DiagnosticReport (+ optionally
  ImagingStudy, Observation per finding).
• DICOM SR — produced via highdicom.sr (MIT, built on pydicom),
  implementing TID 1500 "Measurement Report". Optional, only when DICOM
  evidence images are available.
• Coding — reference RadLex RID and RadElement CDE identifiers in the
  coded content. ⚠️ RadLex / RadReport carry custom RSNA licenses: attribution
  required, identifiers must not be altered, the ontology must not be
  embedded/modified — we only reference IDs, never bundle the ontology.

Where

• New optional extra: interop = ["fhir.resources>=7", "highdicom>=0.22"].
• src/medcheck/pipeline/report.py:
  • generate_fhir_report(ctx) -> str (JSON DiagnosticReport)
  • generate_dicom_sr(ctx) -> str (writes a .dcm SR; requires source DICOM)
  • extend ReportStep.run() to handle report_format in {"fhir", "sr"} and/or
    allow emitting them in addition to the primary format.
• CLI: extend --report choices (pdf|html|json|fhir|sr) and/or add
  --also-emit fhir,sr.
• Map StructureFinding → FHIR Observation / SR content items; carry
  confidence, status, anatomy, and the existing disclaimer/limitations.

Design constraints

• Lazy import of fhir.resources / highdicom; skip with a clear message if
  the interop extra isn't installed.
• DICOM SR requires the original DICOM datasets (study/series/SOP references) —
  only enabled when ingest provided real DICOM; otherwise skip SR and still emit FHIR.
• Keep PDF/HTML/JSON exactly as-is (additive).

Acceptance criteria

• fhir output produces a schema-valid DiagnosticReport JSON (validated by
  fhir.resources model construction).
• sr output writes a TID 1500 SR when source DICOM is available; skips
  gracefully otherwise.
• RadLex/RadElement IDs referenced (not embedded), with attribution noted in docs.
• Lazy imports; no new hard dependencies.
• Tests: FHIR document builds & round-trips; SR built from a mocked/sample
  dataset; formats are additive (PDF/HTML/JSON unaffected).
• Docs: new formats, the interop extra, and the RSNA attribution note.

Effort

~1 day (FHIR first; DICOM SR as a follow-on). Light optional deps.