Add semantic note search (nomic-embed-text) and per-conversation note cache

- New NoteEmbedding model + migration 0014 stores float embeddings (JSONB)
- services/embeddings.py: get_embedding, upsert_note_embedding,
  semantic_search_notes (cosine similarity), backfill_note_embeddings
- build_context() now tries semantic search first, falls back to keyword search;
  accepts cached_note_ids to reuse last-turn notes and stabilise the system
  prompt prefix for Ollama's KV cache
- generation_buffer.py: per-conversation note ID cache (get/set/clear)
- generation_task.py: passes cached IDs into build_context, updates cache
  after each turn, and invalidates it after create_note/update_note/create_task
- app.py: pulls nomic-embed-text at startup and launches a background backfill
  to embed all existing notes (30 s delay so Ollama has time to load the model)
- routes/notes.py + services/tools.py: fire-and-forget embedding update on
  every note create or update via the API or LLM tool calls

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

src/fabledassistant/services/llm.py

@@ -307,6 +307,7 @@ async def build_context(
     user_message: str,
     exclude_note_ids: list[int] | None = None,
     history_summary: str | None = None,
+    cached_note_ids: list[int] | None = None,
 ) -> tuple[list[dict], dict]:
     """Build messages array for Ollama with system prompt and context.
 
@@ -370,29 +371,63 @@ async def build_context(
                 f"--- End Note ---"
             )
 
-    # Search notes by keywords from user message — single OR query
-    keywords = _extract_keywords(user_message)
-    if keywords:
-        search_exclude = set(exclude_set)
-        if current_note_id:
-            search_exclude.add(current_note_id)
+    # Find related notes to inject into context.
+    # Priority: (1) use cached note IDs from a previous turn in this conversation
+    #           (2) try semantic search via nomic-embed-text
+    #           (3) fall back to keyword search
+    # The cache stabilises the system prompt prefix so Ollama's KV cache stays warm.
+    search_exclude = set(exclude_set)
+    if current_note_id:
+        search_exclude.add(current_note_id)
+
+    found_notes = []
+    if cached_note_ids:
+        # Load the same notes as last turn — keeps system prompt prefix identical.
         try:
-            notes = await search_notes_for_context(
-                user_id, keywords, exclude_ids=search_exclude or None, limit=3
-            )
-            snippets: list[str] = []
-            for n in notes:
-                body_preview = n.body[:2000] if n.body else ""
-                snippets.append(f"- {n.title}: {body_preview}")
-                context_meta["auto_notes"].append({"id": n.id, "title": n.title})
-            if snippets:
-                system_parts.append(
-                    "\n\n--- Related Notes ---\n"
-                    + "\n".join(snippets)
-                    + "\n--- End Related Notes ---"
-                )
+            from fabledassistant.services.notes import get_note as _get_note
+            for nid in cached_note_ids:
+                if nid not in search_exclude:
+                    n = await _get_note(user_id, nid)
+                    if n:
+                        found_notes.append(n)
         except Exception:
-            logger.warning("Failed to search notes for context", exc_info=True)
+            logger.warning("Failed to load cached notes for context", exc_info=True)
+            found_notes = []
+
+    if not found_notes:
+        # Try semantic search first; fall back to keyword search on failure / no results.
+        try:
+            from fabledassistant.services.embeddings import semantic_search_notes
+            found_notes = await semantic_search_notes(
+                user_id, user_message, exclude_ids=search_exclude or None, limit=3
+            )
+        except Exception:
+            logger.warning("Semantic note search failed, falling back to keyword search", exc_info=True)
+
+        if not found_notes:
+            keywords = _extract_keywords(user_message)
+            if keywords:
+                try:
+                    found_notes = await search_notes_for_context(
+                        user_id, keywords, exclude_ids=search_exclude or None, limit=3
+                    )
+                except Exception:
+                    logger.warning("Failed to search notes for context", exc_info=True)
+
+    if found_notes:
+        snippets: list[str] = []
+        for n in found_notes:
+            body_preview = n.body[:2000] if n.body else ""
+            snippets.append(f"- {n.title}: {body_preview}")
+            context_meta["auto_notes"].append({"id": n.id, "title": n.title})
+        system_parts.append(
+            "\n\n--- Related Notes ---\n"
+            + "\n".join(snippets)
+            + "\n--- End Related Notes ---"
+        )
+
+    # Expose note IDs so the caller can update the per-conversation cache.
+    context_meta["auto_note_ids"] = [n.id for n in found_notes]
 
     # Fetch URL content from user message
     urls = _find_urls(user_message)