fix(http): fail on non-success response status codes

[allamiro]

Fixes #274.

Problem

The HTTP db backend never checked response status codes. A 404/500 error page was streamed straight into the kdbx parser (reads) or silently treated as a successful upload (writes).

Changes

• get_db_read, get_key_read: error_for_status() before streaming the body
• get_db_write: the upload result channel now reports non-success statuses as errors
• two new regression tests (GET 404, PUT 500), both fail before the fix

Testing

cargo test: 11/11 pass in a rust:1.83 container.

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Summary by cubic

Make the HTTP db backend fail on non-success status codes instead of streaming error pages or treating failed writes as successful. This prevents confusing parser errors and surfaces clear HTTP failures.

• Bug Fixes
  • Use error_for_status() in get_db_read and get_key_read before streaming.
  • Propagate non-2xx statuses as errors in get_db_write.
  • Add regression tests for GET 404 and PUT 500.

^{Written for commit 3c03e80. Summary will update on new commits.}