Stage embeddings: ship Path A (text) now, prepare Path C (text+visual) additively

Decision (Q15 resolved): start with text-only via TEI + bge-m3 in Phase 3.1,
prepare data shapes so Phase 3.2 (visual collection via infinity + jina-clip-v2)
is a pure additive operation — no rename, no schema migration, no n8n rewiring.

Concretely:
- Qdrant collection renamed nexa_knowledge → nexa_knowledge_text (1024-dim
  for bge-m3) with modality-aware payload (modality, source_type, media_uri,
  graph_iri, content_hash, context). Visual placeholder schema committed
  alongside (qdrant_schema_visual.json, 768-dim, jina-clip-v2).
- Image attachments captured in 3.1 are recorded in GraphDB as nexa:Note with
  nexa:modality "image" + nexa:pendingVisualIndex true; the 3.2 backfill
  workflow picks them up and embeds. No data lost between phases — the queue
  is the GraphDB itself.
- RDF schema (docs/08) gains nexa:modality, nexa:mediaUri,
  nexa:vectorCollection, nexa:pendingVisualIndex from day one.
- docs/02 roadmap split: 3.1 = text RAG (Path A), 3.2 = visual collection
  (Path C), 3.4 = Ontotext GraphDB.
- docs/09 grows a "Phase add-on: visual collection (Phase 3.2)" section with
  the TEI→infinity swap, second collection create, LiteLLM second model
  registration, and the SPARQL-driven backfill query.
- New open questions: Q16 (queue ergonomics + does SAIA already proxy an
  embed model?), Q17 (reuse Immich's CLIP for photo-library queries?).
- docs/03 + CLAUDE.md updated so future runs use the new collection names
  and don't re-decide the staging.

nexa-core/config/qdrant_schema_visual.json

@@ -0,0 +1,26 @@
+{
+  "collection_name": "nexa_knowledge_visual",
+  "vector_config": {
+    "size": 768,
+    "distance": "Cosine"
+  },
+  "payload_schema": {
+    "source":      "keyword",
+    "source_type": "keyword",
+    "modality":    "keyword",
+    "mime_type":   "keyword",
+    "media_uri":   "keyword",
+    "context":     "keyword",
+    "tags":        "keyword",
+    "graph_iri":   "keyword",
+    "content_hash":"keyword",
+    "created_at":  "datetime"
+  },
+  "_notes": {
+    "status": "PLACEHOLDER — not deployed in Phase 3.1. Created in Phase 3.2.",
+    "vector_size_rationale": "768 = jina-clip-v2 joint text/image space. The same model embeds both text queries and images, so semantic queries against this collection are simple kNN.",
+    "served_by": "Will be served by `infinity` (https://github.com/michaelfeil/infinity) — drop-in replacement for TEI that supports CLIP-family models with the same OpenAI-compatible API.",
+    "modality_field": "'image' for indexed images, 'text' if a text query has been embedded into the visual space for retrieval (rare — usually we embed the text query at search time and don't store it).",
+    "media_uri_field": "Required for image rows. Examples: 'memos://memos/123/attachment/4', 'nextcloud://files/photos/whiteboard-2025-09.jpg', 'obsidian://attachments/diagram.png'."
+  }
+}