Recovered databases fail `validate --indexed-only`: implement recovered source accounting

[pablontiv]

Split out of #60, which ships the design and plan for this but not the implementation.

Problem

CreateRecoveryDestination installs the canonical union of records while leaving indexed_files empty, and verifyRecoveryDestinationRows currently requires that emptiness:

// internal/storage/recovery_destination.go:243
if indexedFileCount != 0 {
    return fmt.Errorf("indexed_files count = %d, want 0 for recovery destination source accounting", indexedFileCount)
}

Database.Validate requires every search_items.source_path to have an indexed_files row, so a database that recovery just declared healthy immediately fails the public validate --indexed-only orphan check. Recovery's remediation purpose is defeated: the user recovers, then sees the result reported as invalid.

Designed fix (already in the repo)

docs/superpowers/specs/2026-08-19-recovered-source-accounting-design.md and docs/superpowers/plans/2026-08-19-recovered-source-accounting.md specify it:

• One provisional indexed_files row per distinct recovered source_path, hash backscroll:recovered, last_indexed NULL.
• The marker must never look like a 64-char hex SHA-256.
• verifyRecoveryDestinationRows checks an exact marker set instead of zero rows.
• Consumers branch on it: GetFileHashes and GetStats exclude markers; BackfillDerived includes them; SyncFiles' existing INSERT OR REPLACE replaces the marker with the real hash on the next real sync.

Not yet present

recoveredSourceHash, isRecoveredSourceHash, recoveryDestinationSourcePaths, insertRecoveryDestinationAccounting.

Acceptance

A database produced by recover passes validate --indexed-only with no orphan findings, and a later sync replaces every marker with a real hash.

[pablontiv]

Already implemented on main: internal/storage/recovery_accounting.go defines the backscroll:recovered marker, and internal/storage/queries.go plus internal/storage/backfill.go carry the consumer branches. Landed via #59 while this issue was being written.