feat(privacy): detect an already-sanitized source

Scan now reports PrivacyScan.Sanitized {detected, markers, strong, files,
evidence}. SanitizationMarkers recognises a value slot filled with one
repeated placeholder + separators (xxxxx.xxxx.xx, x@xxxx.xxxx.xx,
000.00.00.0, a decoy timezone) - it matches the shape, not the literal "x",
so evolving the redaction mechanism still trips it.

detected requires corroboration: strong>=2, or strong>=1 && markers>=3, or
markers>=4. A single filler-looking token is reported (markers:1) but never
asserted as sanitized, so a partial future pass or a coincidence does not
read as "done". 0.0.0.0 / 000 / UTC / Etc/UTC are too plausibly intentional
and do not count.

UI: the Customer-data panel shows "файл уже обезличен" and hides the
sanitize button when detected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014jDYM1nnoZZ3vFz23DDaV1

bible-local/03-api.md

@@ -155,9 +155,11 @@ Returns normalized parse and collection issues combined from:
 ### `GET /api/privacy-scan`
 
 Returns the customer-data scan for the current dataset (`models.PrivacyScan`:
-`files_scanned`, `customers[]`, `findings[]`, `summary`).
-Returns `{ "loaded": false }` when nothing is loaded or the scan produced
-nothing. Detection only; see `docs/privacy-scan.md`.
+`files_scanned`, `customers[]`, `findings[]`, `summary`), plus `sanitizable`
+(bool) and, when the source already looks de-identified, `sanitized`
+(`{detected, markers, strong, files, evidence}`).
+Returns `{ "loaded": false }` when nothing is loaded. Detection only; see
+`docs/privacy-scan.md`.
 
 ### `POST /api/sanitize`