Switch domain assignment to Qdrant as source of truth

Replace on-disk concept file reads with Qdrant payload queries for
domain assignment. This unlocks assignment for ~10,120 items that had
missing or legacy-only concept files on disk while Qdrant held the
correct 18-domain taxonomy data.

Changes:
- domain_assigner.py: Replace _count_concept_domains (disk) with
  _count_domains_from_qdrant and _count_domains_from_qdrant_batch
  (Qdrant scroll queries). Add _get_qdrant_client helper. Remove
  pass 3 defensive re-run (Qdrant reads are consistent). Add
  no_concepts terminal status for zero-vector documents.
- embedder.py: Post-embed hook passes existing qdrant client to
  compute_assignment, avoiding a second connection.
- recon.py: Backfill creates one QdrantClient for the batch. SQL
  filter includes existing needs_reprocess items. Dry-run reports
  no_concepts as separate bucket. --reprocess-missing removes
  concept-dir deletion step (no longer reads from disk).
- docs/domain-assignment.md: Algorithm references Qdrant, documents
  no_concepts status, removes pass 3 description.

Dry-run results: 20,453 assigned, 1,392 tied, 298 no_concepts,
0 needs_reprocess, 0 errors (previously 10,416 needs_reprocess).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/embedder.py

@@ -411,7 +411,7 @@ def embed_single(file_hash, db, config):
                 from .domain_assigner import compute_assignment
                 from .peertube_writer import push_category, extract_uuid
                 from .recon_domains import DOMAIN_CATEGORY_MAP
-                domain, status = compute_assignment(file_hash, db, config)
+                domain, status = compute_assignment(file_hash, db, config, qdrant=qdrant)
                 db.set_domain_assignment(file_hash, domain, status)
                 if domain and status == 'assigned':
                     cat_id = DOMAIN_CATEGORY_MAP[domain]