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/generation_buffer.py

@@ -74,6 +74,27 @@ class GenerationBuffer:
 
 # Module-level singleton registry
 _buffers: dict[int | str, GenerationBuffer] = {}
+
+# Per-conversation note context cache — maps conv_id → sorted list of note IDs.
+# Stores the note IDs that were last included in the system prompt so that
+# subsequent turns in the same conversation can reuse them, stabilizing the
+# system prompt prefix and improving Ollama's KV cache hit rate.
+_conv_note_cache: dict[int, list[int]] = {}
+
+
+def get_conv_note_cache(conv_id: int) -> list[int]:
+    """Return cached note IDs for a conversation (empty list if none)."""
+    return list(_conv_note_cache.get(conv_id, []))
+
+
+def set_conv_note_cache(conv_id: int, note_ids: list[int]) -> None:
+    """Store note IDs to reuse on the next turn of this conversation."""
+    _conv_note_cache[conv_id] = list(note_ids)
+
+
+def clear_conv_note_cache(conv_id: int) -> None:
+    """Invalidate the note cache for a conversation (e.g. after a note write)."""
+    _conv_note_cache.pop(conv_id, None)
 _cleanup_task: asyncio.Task | None = None
 _GRACE_PERIOD = 60.0  # seconds to keep completed buffers