fix(settings): whole-line path redaction + value validation + full schema counts (R2-P1-4/P2-3/P2-4)

R2-P1-4: replace the token-based redaction with a Redactor that scrubs known
source roots + home dir + username (exact substrings, incl. spaces), then an
absolute-path-run scanner handling key=path, quoted, spaces, and UNC shapes,
then the residual token scrub - so user paths/filenames no longer leak into the
exported bundle. No new dependency (hand-written scanner).

R2-P2-3: validate settings values backend-side before store_group - numeric
ranges (scan/deep-verify/check intervals, bandwidth cap, concurrency) + enums
(io_priority, log_level, channel, color_mode, tray target, vss_mode) + a BCP-47
locale shape check -> internal.invalid_input.

R2-P2-4: schema.txt counts every KNOWN_STATE_TABLES table, not just accounts +
backup_sources.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CyiRqk2DVwmJjEu5gcD1m

Author: pmaxhogan