feat(rag): RAG scoping and context isolation controls

- Migration 0030: add conversations.rag_project_id (NULL=orphan-only,
  -1=all notes, positive=project), projects.auto_summary and
  projects.summary_updated_at
- Three-value scope semantics thread from build_context() → semantic
  search and keyword fallback via orphan_only + effective_project_id
- Project summarization background job (generate_project_summary,
  backfill_project_summaries) called via Ollama; triggered on project
  update and note saves (debounced 1h); runs at startup
- New LLM tools: search_projects (SequenceMatcher scoring on
  title+description+auto_summary) and set_rag_scope (persists to DB,
  workspace-guarded, emits new_rag_scope in SSE done event)
- execute_tool() accepts conv_id + workspace_project_id; generation_task
  passes both and captures scope changes for SSE done enrichment
- Frontend: Conversation type gets rag_project_id; chat store adds
  ragProjectId computed + updateRagScope(); SSE done handler syncs scope
- ChatView: replace sidebar ProjectSelector with a scope chip pill above
  the input bar, animated dropdown, pulse on model-driven scope change

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

src/fabledassistant/services/chat.py

@@ -139,11 +139,15 @@ async def cleanup_old_conversations(user_id: int, days: int) -> int:
         return len(result.fetchall())
 
 
+_UNSET = object()
+
+
 async def update_conversation(
     user_id: int,
     conversation_id: int,
     title: str | None = None,
     model: str | None = None,
+    rag_project_id: object = _UNSET,
 ) -> Conversation | None:
     async with async_session() as session:
         result = await session.execute(
@@ -159,6 +163,8 @@ async def update_conversation(
             conv.title = title
         if model is not None:
             conv.model = model
+        if rag_project_id is not _UNSET:
+            conv.rag_project_id = rag_project_id  # type: ignore[assignment]
         conv.updated_at = datetime.now(timezone.utc)
         await session.commit()
         await session.refresh(conv)