Add pension PDF upload, parsing, and fund summary API

Backend: parse BAC pension statement PDFs (VOL, ROP, FCL) via
pdftotext, store snapshots with duplicate detection, reject
credit card statements. Endpoints: POST /upload, GET /snapshots,
GET /fund-summary.

Frontend: wire up drag-and-drop upload, load real balances and
rendimientos from API, show upload results with error/duplicate
feedback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -1,5 +1,6 @@
 FROM python:3.11-slim
 WORKDIR /app
+RUN apt-get update && apt-get install -y --no-install-recommends poppler-utils && rm -rf /var/lib/apt/lists/*
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 COPY . .