fix(backend): write full ODM note only on first column, short marker on the rest

[laskevych]

Preview

Summary

When a Data Mart is imported into a Google Sheet, the full "Imported via OWOX
Data Marts" provenance block (timestamp, Data Mart name, Data Mart page link)
was duplicated into the note of every column header. Combined with the
per-column description, this buried the user's own column descriptions under
repeated technical metadata.

This change keeps the ODM ownership signal on every imported column while
showing the full metadata block only once:

• First column of the range (A1): full note — the column description, a
  blank line, then the ODM metadata block (--- Imported via OWOX Data Marts ---,
  import timestamp, Data Mart name, Data Mart page link).
• All other columns: only the short marker --- Imported via OWOX Data Marts ---,
  enough to show the column is ODM-managed without repeating the metadata.
• Joined / blended Data Marts: A1 references only the main (Original/Home)
  Data Mart. This already held — report.dataMart is always the main mart, and
  joinable marts only affect the column alias, never the note — and is now
  documented in the writer.

Changes

• sheet-metadata-formatter.ts
  • Add buildImportedColumnMarker(isCommunityEdition) returning the short marker.
  • Extract private buildOdmMarker() so the full note and the short marker
    read identically.
  • Restructure buildImportedColumnNote(): the marker is now the header line
    of the metadata block (Imported at <timestamp> instead of
    Imported via OWOX Data Marts at <timestamp>), and the description is
    separated from the block by a blank line so the two never read as one
    paragraph.
• google-sheets-report-writer.ts
  • In writeHeaders(), branch on idx === 0: the first column gets the full
    note, every other column gets the short marker.

Notes

• The change only affects the text of the notes. ODM-owned-column
  detection and imported-range delineation rely on developer metadata
  (OWOX_COLUMNS / OWOX_REPORT_META), not on note text, so behavior is
  unaffected.
• Existing sheets keep their old per-column notes until the next refresh, which
  rewrites all header notes — they self-heal on the next import.

Testing

• Updated sheet-metadata-formatter.spec.ts for the new note format and added
  coverage for buildImportedColumnMarker.
• Added a writer test asserting exactly one full note (A1) + N−1 short markers.
• All 29 tests pass; lint clean on the changed files.

DoD

• A1 of the Data Mart range carries the full note: column description + ODM
  metadata block (timestamp, Data Mart name, Data Mart page link)
• Other columns of the range carry only the short marker
  (--- Imported via OWOX Data Marts ---)
• For joined Data Marts, A1 references only the main (Original/Home) Data
  Mart, without joinable marts
• Column description is visually separated from the ODM marker (blank line
  between them)