feat: glob-based overlay discovery via OVERLAY_PATTERN

[PABannier]

Summary

• Add OVERLAY_PATTERN env var to discover overlay files across arbitrary nested directory structures using glob patterns
• At startup, the server scans matching paths and builds a HashMap<String, PathBuf> index (slide ID → overlay file path)
• Strips known slide extensions (.svs, .ndpi, etc.) from parent directory names so overlay IDs match slide IDs
• When OVERLAY_PATTERN is unset, existing OVERLAY_DIR behavior is preserved unchanged

Usage

docker run -p 8080:8080 \
  -v ~/slides:/slides \
  -v /data/cell_masks:/overlays \
  -e OVERLAY_PATTERN="/overlays/**/*/cell_masks.bin" \
  pathcollab:latest

Test plan

• cargo check compiles
• cargo test — all 65 tests pass (28 unit + 37 integration)
• Manual: Docker image built and tested with real cell masks directory

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