Shorten per-row Source File paths to extraction-relative (follow-up to #931)

[abrignoni]

Summary

Part 2 of the source-path fix. #931 shortens the returned source_path centrally in artifact_processor, but as @JamesHabben pointed out, artifacts that drop the path into a per-record Source File column are a separate population the decorator can't reach. This fixes all of them.

Audit method: for every artifact function, locate data_headers entries matching Source File/Path/DB, resolve which appended expression feeds that column index, and classify. Result: 26 flagged → 17 already shortened → 19 true positives in 7 files, all fixed here by wrapping the row-inserted copy with Context.get_relative_path() (the absolute path is still used for file/db access):

• Life360 (chat messages, places, locations, device battery)
• SimpleStorage_applaunch
• battery_usage_v9 (both artifacts)
• gmailIMAPEmails (emails + accounts)
• googleChat (all 4 artifacts — this was the leak originally observed in test output)
• knuddels (chats, contacts, account — including the active_row sources join)
• thunderbird

Verified false positives, deliberately untouched:

• FacebookMessenger — already shortens via its module-level _src() helper
• discreteNative — emits Path(file).name only
• honorMediaLibrary — its Source File Path/Name columns are device-side paths from gallery.db (evidence data, not examiner paths)

Testing

• Re-ran the audit after the fix: zero unshortened row-level emitters remain (only the three false positives above)
• pylint 10.00/10 on all 7 files; byte-compile; PluginLoader registers all 583 plugins
• get_relative_path passthrough semantics make every wrap safe when data_folder is unset