feat: Tink open banking demo — 6-step API walkthrough

Demonstrates the full Tink integration flow for open banking:
  Step 1 — Client credentials auth (app token)
  Step 2 — Create Tink user with external_user_id
  Step 3 — Connect bank via Tink Link OAuth redirect
  Step 4 — List accounts (v2 endpoint)
  Step 5 — List transactions (v2 endpoint, cursor pagination)
  Step 6 — Webhooks (register endpoint, receive events)

Built with Python / FastAPI + Jinja2 templates.
Each step shows live JSON responses, cURL examples and API version badges.
Includes server-side token store (prevents session cookie overflow),
asyncio lock on OAuth callback, and demo mode with realistic mock data.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker-compose.yml

@@ -0,0 +1,10 @@
+version: "3.9"
+services:
+  demo:
+    build: .
+    ports:
+      - "8000:8000"
+    env_file: .env
+    volumes:
+      - ./src:/app/src  # hot reload mount
+    command: uvicorn src.main:app --host 0.0.0.0 --port 8000 --reload