Refactor: extract _image_cache_key helper and drop stale response_model on get_nearby_issues

[Copilot]

Description

Addresses two review comments from PR #573: eliminates duplicated MD5 cache-key construction in detection.py and removes a misleading response_model annotation that diverged from the actual raw-Response return type.

Changes

• backend/routers/detection.py: Extracted _image_cache_key(prefix, image_bytes) -> str helper. All 8 _cached_* functions now delegate to it instead of inlining hashlib.md5(image_bytes).hexdigest() with a manual f-string each time.

  # Before (repeated 8×)
  image_hash = hashlib.md5(image_bytes).hexdigest()
  key = f"severity_{image_hash}"
  return await _get_cached_result(key, detect_severity_clip, image_bytes)
  
  # After
  def _image_cache_key(prefix: str, image_bytes: bytes) -> str:
      return f"{prefix}_{hashlib.md5(image_bytes).hexdigest()}"
  
  async def _cached_detect_severity(image_bytes: bytes):
      return await _get_cached_result(_image_cache_key("severity", image_bytes), detect_severity_clip, image_bytes)

• backend/routers/issues.py: Removed response_model=List[NearbyIssueResponse] from get_nearby_issues. The handler returns a pre-serialised fastapi.Response; keeping the annotation caused the generated OpenAPI schema to misrepresent the actual contract and bypassed no-op Pydantic validation.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🎨 Code style update (formatting, renaming)
• ♻️ Refactoring (no functional changes)
• ⚡ Performance improvement
• ✅ Test update

Related Issue

Closes #

Testing Done

• Tested locally
• Added/updated tests
• All tests passing

Screenshots (if applicable)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Co-Authors

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Summary by cubic

Consolidates image cache key generation into a single helper for deterministic keys and fewer cache misses. Also fixes a wrong response_model on the nearby issues route and a small indentation error.

• Cache Stability

  • Extracted _image_cache_key(prefix, image_bytes) and used it across all 8 _cached_* functions for consistent MD5-based keys.

• Bug Fixes

  • Removed incorrect response_model from GET /issues/nearby since it returns a raw Response.
  • Fixed an indentation issue that could cause parse/runtime errors.

^{Written for commit fea1bed. Summary will update on new commits.}