WEBDEV-9423: Migrate metadata-service into elements

[jbuckner]

Moves metadata-service into src/services/metadata-service/, ported from @internetarchive/metadata-service 1.1.1 (tag v1.1.1, same as the published source). All 15 tests came over with the assertions unchanged. It imports the local item-metadata and result-type. MetadataServiceErrorType is a const object with a same-named type and the same values, because the repo forbids enums. The entry point is index.ts, since metadata-service.ts is the class. The open upstream writes PR (metadata-service#20, WEBDEV-9151) is not included.

Stacked on #171 (item-metadata). The branch also carries #156 (result-type) until it merges.

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https://claude.ai/code/session_01CjuFdHEwY7MowcqkUxxudu

[github-actions]

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[jbuckner]

Second commit is a security fix beyond the port. DefaultMetadataBackend.fetchMetadata put the identifier and key path into the URL unencoded, so ../services/user or x?y=1 could change which archive.org URL gets fetched (same in the published 1.1.1). Each part is encoded now and . / .. parts return an itemNotFound error, with tests. Worth fixing upstream too.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.15663% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.67%. Comparing base (db7d0df) to head (8708e82).

Files with missing lines	Patch %	Lines
...tadata-service/backend/default-metadata-backend.ts	80.48%	4 Missing and 4 partials ⚠️
...es/metadata-service/responses/metadata-response.ts	95.23%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           WEBDEV-9419-item-metadata     #174      +/-   ##
=============================================================
- Coverage                      91.73%   91.67%   -0.06%     
=============================================================
  Files                            126      131       +5     
  Lines                           4075     4158      +83     
  Branches                         858      876      +18     
=============================================================
+ Hits                            3738     3812      +74     
- Misses                           147      152       +5     
- Partials                         190      194       +4     

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