MISC changes

Fail hard on registry fetch errors, include stub entries for unparseable mapping sets, and rename fair_score to metadata_completeness_score

- Registry-level fetch failures (e.g. HTTP 403 from GitLab) now abort the
  pipeline instead of silently dropping all mapping sets from that registry.
- All HTTP requests use a shared session with a descriptive User-Agent header
  to avoid being blocked by hosts like GitLab.
- Mapping sets that fail to parse are no longer silently skipped. Instead,
  stub entries are included in the output with a status field:
  ok, no_metadata, nonstandard_format, or fetch_error.
- Renamed fair_score to metadata_completeness_score throughout the codebase
  (CLI, frontend JS, CSS). Tooltip now explains the score indicates how rich
  the metadata provided by the source is.
- Added ebi-text-mappings registry to mapping-server.yml.
- Removed the word "FAIR" from user-facing UI text (tagline, score labels).

Author: matentzn