* feat: allow per-request file parser selection with fallback chains
Clients can now specify which parser(s) to use when calling the file
retain endpoint, instead of being locked to the server-side default.
Changes:
- `parser` field added to `FileRetainRequest` (request-level default)
and `FileRetainMetadata` (per-file override); accepts a single name
or an ordered fallback chain (list)
- Resolution priority: per-file > request-level > server default
- `HINDSIGHT_API_FILE_PARSER` now accepts a comma-separated fallback
chain (e.g. `iris,markitdown`); fully backward-compatible
- New `HINDSIGHT_API_FILE_PARSER_ALLOWLIST` env var restricts which
parsers clients may request (defaults to all registered parsers)
- Invalid/disallowed parser names are rejected with HTTP 400
- `FileParserRegistry.convert_with_fallback()` tries each parser in
order, falling back on UnsupportedFileTypeError, empty content, or
any other error
- Worker updated to use the fallback chain stored per-task
- OpenAPI spec and all generated clients regenerated
* fix: handle on_file_convert_complete hook and rebase onto main
- Return ConvertResult dataclass from convert_with_fallback() instead
of a plain str, carrying both the content and the winning parser name
- Use winning_parser_name in the on_file_convert_complete hook so
parser_name reflects the parser that actually succeeded, not the chain
- Update all test calls to submit_async_file_retain() to use the new
per-item parser field instead of the removed top-level parser= kwarg
* docs: document HINDSIGHT_API_FILE_PARSER fallback chain and ALLOWLIST
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