feat: add t-SNE stock clustering and similarity search (TDD)

2 new endpoints:
- POST /portfolio/cluster - t-SNE + KMeans clustering by return
  similarity. Maps stocks to 2D coordinates with cluster labels.
- POST /portfolio/similar - find most/least similar stocks by
  return correlation against a target symbol.

Implementation:
- sklearn TSNE (method=exact) + KMeans with auto n_clusters
- Jitter handling for identical returns edge case
- 33 new tests (17 service unit + 16 route integration)
- All 503 tests passing

routes_portfolio.py

@@ -52,3 +52,45 @@ async def portfolio_risk_parity(request: PortfolioOptimizeRequest):
     except ValueError as exc:
         raise HTTPException(status_code=400, detail=str(exc))
     return ApiResponse(data=result)
+
+
+class ClusterRequest(BaseModel):
+    symbols: list[str] = Field(..., min_length=3, max_length=50)
+    days: int = Field(default=180, ge=30, le=3650)
+    n_clusters: int | None = Field(default=None, ge=2, le=20)
+
+
+class SimilarRequest(BaseModel):
+    symbol: str = Field(..., min_length=1, max_length=20)
+    universe: list[str] = Field(..., min_length=2, max_length=50)
+    days: int = Field(default=180, ge=30, le=3650)
+    top_n: int = Field(default=5, ge=1, le=20)
+
+
+@router.post("/cluster", response_model=ApiResponse)
+@safe
+async def portfolio_cluster(request: ClusterRequest):
+    """Cluster stocks by return similarity using t-SNE + KMeans."""
+    try:
+        result = await portfolio_service.cluster_stocks(
+            request.symbols, days=request.days, n_clusters=request.n_clusters
+        )
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    return ApiResponse(data=result)
+
+
+@router.post("/similar", response_model=ApiResponse)
+@safe
+async def portfolio_similar(request: SimilarRequest):
+    """Find stocks most/least similar to a target by return correlation."""
+    try:
+        result = await portfolio_service.find_similar_stocks(
+            request.symbol,
+            request.universe,
+            days=request.days,
+            top_n=request.top_n,
+        )
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc))
+    return ApiResponse(data=result)