LibriVox add: identity edges after adopted #385

Follow-ups from v2.7.0 batch B (#384, KodeStar/audiosilo-bridge#70, KodeStar/audiosilo-frontend#297).

Batch B made the librivox.fetch fold respect an admin's identity: the Bridge's result now carries adopted (bridge internal/librivoxjob/librivoxjob.go:115, always sent), the server reads it as a tri-state (internal/catalog/librivox.go:63), an adopted folder never overwrites an existing book_matches / book_overrides identity, and a newly placed book drops a deleted book's leftover ASIN/ISBN override and enrichment row at its path. These edges are left (they continue #379, which the PR closes).

• Placement time on the wire (server and Bridge). After the Bridge fix round, adopted is false only when this run renamed staging into place. A run that renames, then fails its rescan, retries with adopted: true, so a deleted earlier book's overrides, enrichment and match at that path survive. Fix: the Bridge records the placement time in its fetch state and sends it as an additive result field; the fold drops only rows older than it. Needs a CROSS-REPO §23 line; the Bridge half is tracked in librivox.fetch: pairing, sweep and lane edges audiosilo-bridge#71.
• Same-work match upgrade (owner decision). An adopted fold never upgrades a same-work match that has no recording (for example a bulk match run's), recorded as a known limit in CONVENTIONS.md. Owner to decide whether a same-work how=auto match should be upgraded to the fetched recording.
• A placed book inherits a deleted book's other leftovers. Only ASIN/ISBN overrides and the enrichment row are dropped today; title and other field overrides and a custom cover at the same path still apply to the new book.
• Rebuild identity from .audiosilo/librivox.json (v1) after Clear community matches or a lost fold. Needs the scanner/index to read the file, so it was left out of batch B.
• A fourth librivox_match value. The unmatched notification says "It is in the library now" even when no book was indexed at the path. A separate value (matched, kept, unmatched, plus one for placed but not indexed) with its own wording would keep the notification honest. Additive on the notification event.