feat(download): add minimal DRS 1.5 object endpoint

[jhagberg]

Related issue(s) and PR(s)

This PR closes #2246 (SDA-side work only; htsget-rs DrsStorage contribution is separate).

Related: umccr/htsget-rs#356

Description

Adds GET /objects/{datasetId}/{filePath} — a minimal GA4GH DRS 1.5 endpoint that resolves a dataset + file path to a DrsObject with a pre-resolved access_url pointing to /files/{fileId}/content.

Changes:

• Database: Added CreatedAt to File struct, new GetFileChecksums method for querying checksums by source (ARCHIVED/UNENCRYPTED)
• Handler: handlers/drs.go — path parsing, auth, file resolution, DRS response construction
• Route: GET /objects/*path registered with auth middleware
• Swagger: Endpoint + DRS schemas (DrsObject, DrsChecksum, DrsAccessMethod, DrsAccessURL)

DRS 1.5 compliance:

• size = ArchiveSize (encrypted blob size per spec: "blob size in bytes")
• checksums from ARCHIVED source (per spec: "computed over the bytes in the blob")
• self_uri, created_time, access_methods all per DRS 1.5 required fields

Note: The htsget-rs maintainer pivoted to a ResolveStorage backend using the existing /datasets/{id}/files?filePath= endpoint. This DRS endpoint is an independent, spec-compliant addition with value for any DRS-aware tooling.

ADR

• This PR includes an architecturally significant decision (see docs/decisions/README.md)
  • A decision record has been added or updated in docs/decisions/

How to test

cd sda && go test ./cmd/download/... -count=1
cd sda && go test -tags visas -count=1 ./cmd/download/...

10 unit tests covering: success path, 403 (not found + no access), 400 (malformed paths), 401, 500, empty checksums, multiple checksums, checksum type normalization. Route wiring test verifies auth requirement.

[codecov]

Codecov Report

❌ Patch coverage is 72.94118% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.50%. Comparing base (bca6183) to head (eecfa9c).

Files with missing lines	Patch %	Lines
sda/cmd/download/database/database.go	10.52%	17 Missing ⚠️
sda/cmd/download/handlers/drs.go	93.44%	3 Missing and 1 partial ⚠️
sda/cmd/download/database/cache.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           feature/sda-download-v2    #2296      +/-   ##
===========================================================
+ Coverage                    42.29%   42.50%   +0.21%     
===========================================================
  Files                          120      121       +1     
  Lines                        12242    12327      +85     
===========================================================
+ Hits                          5178     5240      +62     
- Misses                        6432     6454      +22     
- Partials                       632      633       +1     

Flag	Coverage Δ
unittests	42.50% <72.94%> (+0.21%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sda/cmd/download/handlers/handlers.go	72.22% <100.00%> (+1.63%)	⬆️
sda/cmd/download/database/cache.go	77.63% <0.00%> (-1.04%)	⬇️
sda/cmd/download/handlers/drs.go	93.44% <93.44%> (ø)
sda/cmd/download/database/database.go	64.51% <10.52%> (-3.53%)	⬇️

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