feat(holdings): REQ-043 현재 보유 평단(WAC) read — GET /v1/portfolios/{id}/holdings/cost-basis (BAT-018, PR #14)

dcy-008(confirmed)/REQ-043(Approved)/BAT-018. trex-core dcy-034 FR-7 cold-start seed 의존. AC 11/11, QC PASS. worktree+PR 워크플로 첫 파일럿.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

app/schemas/v1.py

@@ -296,3 +296,17 @@ class RealizedPnlResponse(BaseModel):
     items: list[RealizedPnlItem]
     cost_basis_from: datetime | None = None
     data_completeness_warning: str | None = None
+
+
+# ---------------------------------------------------------------------------
+# GET /v1/portfolios/{id}/holdings/cost-basis (REQ-043)
+# ---------------------------------------------------------------------------
+
+class HoldingsCostBasisResponse(BaseModel):
+    portfolio_id: str
+    symbol: str | None = None
+    current_quantity: float
+    cost_basis_per_unit: float | None = None
+    total_cost_basis: float | None = None
+    cost_basis_from: datetime | None = None
+    data_completeness_warning: str | None = None