phase B (2/2): CSV upload endpoint + drag-drop UI

Completes Phase B. The full alternative-onboarding flow is now end-to-end:
drop a T212 pie CSV → parser → InstrumentMap resolver → PortfolioSnapshot
+ Position rows, all without ever asking the user for broker credentials.

- persist_pie() in app/services/csv_import.py: takes a ParsedPie, resolves
  each Slice via InstrumentMap, writes Portfolio + Snapshot + Position
  rows. Unmapped slices are still persisted using their CSV values and
  surfaced in the response for the UI to warn about.
- POST /api/portfolios/upload: multipart endpoint accepting CSV file +
  optional portfolio_name + currency. 2 MiB cap. Returns import summary.
- /upload page with drag-drop dropzone, file input fallback, and inline
  result panel showing invested/value/result + unmapped-slice warnings.
- New "Import" link in the header nav.

Verified end-to-end against the real T212 export: all 13 positions land
with correct T212 tickers (incl. FPp_EQ for the Paris TotalEnergies
listing the heuristic resolver picks), zero unmapped slices, totals
reconcile to the penny.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routers/api.py

@@ -9,7 +9,7 @@ import calendar as _cal
 import re
 from datetime import date, datetime, timedelta, timezone
 
-from fastapi import APIRouter, Depends, HTTPException, Query, Request
+from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, Request, UploadFile
 from fastapi.responses import HTMLResponse, JSONResponse
 from sqlalchemy import desc, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -383,6 +383,66 @@ async def log_days(
 # --- Portfolios --------------------------------------------------------------
 
 
+# 2 MiB max for CSV uploads — T212 pies don't exceed a few KB in practice.
+# Keeps the abuse vector small without rejecting legitimate exports.
+_MAX_CSV_BYTES = 2 * 1024 * 1024
+
+
+@router.post("/portfolios/upload")
+async def upload_portfolio_csv(
+    file: UploadFile = File(...),
+    portfolio_name: str | None = Form(default=None),
+    currency: str = Form(default="GBP"),
+    session: AsyncSession = Depends(get_session),
+):
+    """Import a Trading 212 pie-export CSV. Parses, resolves each Slice to a
+    T212 ticker + Yahoo symbol via InstrumentMap, and persists a new
+    PortfolioSnapshot + Position rows.
+
+    No user-id scoping yet — that lands in phase C. Until then, all uploads
+    land in the single shared portfolio identified by name."""
+    from app.services.csv_import import CSVImportError, parse_t212_csv, persist_pie
+
+    if not file.filename:
+        raise HTTPException(status_code=400, detail="No file uploaded")
+    if not file.filename.lower().endswith(".csv"):
+        raise HTTPException(status_code=400, detail="File must have .csv extension")
+
+    raw = await file.read(_MAX_CSV_BYTES + 1)
+    if len(raw) > _MAX_CSV_BYTES:
+        raise HTTPException(status_code=413, detail=f"File exceeds {_MAX_CSV_BYTES} bytes")
+    if not raw:
+        raise HTTPException(status_code=400, detail="File is empty")
+
+    try:
+        pie = parse_t212_csv(raw)
+    except CSVImportError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+
+    try:
+        result = await persist_pie(
+            session, pie,
+            portfolio_name=portfolio_name,
+            currency=currency,
+        )
+    except Exception as e:
+        # Roll back; surface a clean error
+        await session.rollback()
+        raise HTTPException(status_code=500, detail=f"Persist failed: {e}")
+
+    return {
+        "portfolio_id": result.portfolio_id,
+        "snapshot_id": result.snapshot_id,
+        "portfolio_name": result.portfolio_name,
+        "is_new_portfolio": result.is_new_portfolio,
+        "positions": result.positions_written,
+        "unmapped": result.unmapped_slices,
+        "invested": pie.invested,
+        "value": pie.value,
+        "result": pie.result,
+    }
+
+
 @router.get("/portfolios")
 async def portfolios(
     request: Request,