Studio: add RAG with hybrid search, reranker, chat integration

Backend (studio/backend/):
- core/rag/: parsers (PDF/TXT/MD/DOCX/HTML via pypdf/python-docx/bs4),
  recursive token-aware chunker, embeddings singleton via
  FastSentenceTransformer.from_pretrained(for_inference=True), Qdrant
  local vector store, bm25s lexical index, RRF hybrid retrieval,
  spawn-subprocess ingestion job with SSE progress, optional
  CrossEncoder reranker (off-by-default).
- routes/rag.py: KB CRUD, doc upload (KB + per-thread), doc list/delete,
  ingestion SSE, hybrid+rerank search, thread-index list/clear.
- routes/chat_history.py: purge thread RAG artifacts on thread delete
  and clear-all (rag_documents has no FK cascade to chat_threads so
  uploads work on un-persisted threads).
- studio.db gains 4 RAG tables; storage_roots gains rag_*() helpers.
- auth/authentication.py: get_current_subject_sse accepts ?token=... so
  EventSource can stream ingestion progress.

Frontend (studio/frontend/):
- features/rag/: api client, Zustand store, hooks, dropzone, KB list,
  doc rows, ingestion-progress, thread-index list components.
- Settings dialog gains a Knowledge Bases tab (master/detail + thread
  documents list); /knowledge-bases deep-links to it.
- features/chat/: per-thread ragSource/enableRerank/ragTopK state in
  chat-runtime-store; Retrieval section in chat-settings-sheet with KB
  DropdownMenu (active highlight + per-row trash), thread doc list with
  Clear-thread-index button, RAG Top K slider, reranker toggle;
  chat-adapter retrieves before /v1/chat/completions and injects hits
  as a system block; shared-composer + button routes documents into
  pendingDocs (auto-uploads, send blocked while indexing).

tests/python/test_rag_bm25.py

@@ -0,0 +1,69 @@
+"""BM25 index lifecycle tests (skipped if bm25s is unavailable)."""
+
+import sys
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+STUDIO_BACKEND = REPO_ROOT / "studio" / "backend"
+if str(STUDIO_BACKEND) not in sys.path:
+    sys.path.insert(0, str(STUDIO_BACKEND))
+
+pytest.importorskip("bm25s")
+
+
+@pytest.fixture
+def isolated_bm25_root(tmp_path, monkeypatch):
+    from utils.paths import storage_roots
+
+    monkeypatch.setenv("UNSLOTH_STUDIO_HOME", str(tmp_path))
+    # Reset module-level cache between tests.
+    from core.rag import bm25
+
+    bm25._cache.clear()
+    return tmp_path
+
+
+def test_bm25_index_search_roundtrip(isolated_bm25_root):
+    from core.rag import bm25
+
+    scope = "kb_test"
+    chunks = [
+        {"id": "c1", "text": "the quick brown fox jumps over the lazy dog"},
+        {"id": "c2", "text": "machine learning models predict outputs from inputs"},
+        {"id": "c3", "text": "fox terriers are small dogs"},
+    ]
+    bm25.rebuild_index(scope, chunks)
+    results = bm25.search(scope, "fox", k = 3)
+    ids = [cid for cid, _ in results]
+    assert "c1" in ids
+    assert "c3" in ids
+
+
+def test_bm25_empty_returns_empty(isolated_bm25_root):
+    from core.rag import bm25
+
+    assert bm25.search("kb_nonexistent", "anything", k = 5) == []
+
+
+def test_bm25_delete_scope(isolated_bm25_root):
+    from core.rag import bm25
+
+    scope = "kb_del"
+    chunks = [{"id": "a", "text": "alpha beta gamma"}]
+    bm25.rebuild_index(scope, chunks)
+    assert bm25.search(scope, "alpha", k = 1)
+    bm25.delete_scope(scope)
+    assert bm25.search(scope, "alpha", k = 1) == []
+
+
+def test_bm25_rebuild_replaces_old_corpus(isolated_bm25_root):
+    from core.rag import bm25
+
+    scope = "kb_replace"
+    bm25.rebuild_index(scope, [{"id": "old", "text": "alpha beta"}])
+    bm25.rebuild_index(scope, [{"id": "new", "text": "gamma delta"}])
+    results = bm25.search(scope, "alpha", k = 5)
+    ids = [cid for cid, _ in results]
+    assert "old" not in ids