fix(nzbdav): register migrated files in file_health on symlink match

[javi11]

Summary

• POST /import/nzbdav/migrate-symlinks rewrote library symlinks on disk but never registered the resulting files in file_health — the table the VFS and health checker rely on — so the health system had no record of migrated files.
• Same call site also never advanced import_migrations.status from imported → symlinks_migrated, so the UI's migration_stats.symlinks_migrated counter stayed at zero.

What changed

• SymlinkLookup is now Resolve + CommitRewrites. The walker in RewriteLibrarySymlinks collects every successfully rewritten item and calls CommitRewrites once at the end of the walk (skipped in dry-run). Commit errors are reported in the report but not propagated as a fatal walk error — the filesystem rewrites already happened.
• DBSymlinkLookup now holds *HealthRepository, the queue *Repository (used to resolve source_nzb_path via queue_item_id), and a ConfigGetter. CommitRewrites bulk-advances migration status via the existing MarkSymlinksMigrated, then upserts a file_health row per item via the same AddFileToHealthCheck call the regular post-processor uses (health_scheduler.go). Individual AddFileToHealthCheck failures are joined via errors.Join, never aborted.
• handleMigrateNzbdavSymlinks passes s.healthRepo, s.queueRepo, and s.configManager.GetConfigGetter() to NewDBSymlinkLookup, plus a nil-check for healthRepo.

Reviewer notes

• No schema changes — both file_health (upsert via ON CONFLICT(file_path)) and import_migrations (MarkSymlinksMigrated is a bulk WHERE id IN (...)) already supported this; re-running migrate-symlinks is idempotent.
• Phase 1 import path (processBatch and queue processing) is untouched. Files imported via Phase 1 already get file_health rows from the post-processor's ScheduleHealthCheck — no regression risk there.
• dry_run remains a pure preview: CommitRewrites is not invoked.

Test plan

• go build ./... green
• go vet ./... clean
• go test -race ./internal/importer/migration/... ./internal/database/... ./internal/api/... ./internal/importer/... all green
• Unit test updated to assert CommitRewrites is called once with the expected RowID set on success and not at all in dry-run
• Manual end-to-end:
  • Run POST /import/nzbdav → wait for completed
  • Run POST /import/nzbdav/migrate-symlinks against a library containing symlinks targeting the nzbdav .ids/ directory
  • Confirm SELECT status, COUNT(*) FROM import_migrations GROUP BY status shows non-zero symlinks_migrated
  • Confirm SELECT COUNT(*) FROM file_health WHERE file_path IN (<rewritten finalPaths>) matches report.rewritten
  • Re-run migrate-symlinks; second pass should be a no-op (no duplicate file_health rows, status already symlinks_migrated)
  • Re-run with dry_run: true; no new rows in either table