fix(deps): add flagembedding>=1.2 for BGE-M3 hybrid encode (ADR-0002)

SentenceTransformer.encode() lacks return_dense/return_sparse kwargs;
BGEM3FlagModel provides the correct hybrid API.

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

pyproject.toml

@@ -13,6 +13,7 @@ dependencies = [
     "pgvector>=0.3",
     "pydantic-settings>=2",
     "sentence-transformers>=3",
+    "flagembedding>=1.2",
     "typer>=0.12",
     "httpx>=0.27",
     "tenacity>=8",