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>

src/config.py

@@ -0,0 +1,25 @@
+"""
+App configuration loaded from environment / .env file.
+"""
+
+import os
+from functools import lru_cache
+from dotenv import load_dotenv
+
+load_dotenv()
+
+
+class Settings:
+    tink_client_id: str = os.environ["TINK_CLIENT_ID"]
+    tink_client_secret: str = os.environ["TINK_CLIENT_SECRET"]
+    tink_redirect_uri: str = os.getenv("TINK_REDIRECT_URI", "http://localhost:8000/callback")
+    app_base_url: str = os.getenv("APP_BASE_URL", "http://localhost:8000")
+    session_secret: str = os.getenv("SESSION_SECRET", "dev-only-change-in-prod")
+    tink_api_base: str = os.getenv("TINK_API_BASE", "https://api.tink.com")
+    tink_link_base: str = os.getenv("TINK_LINK_BASE", "https://link.tink.com")
+    demo_mode: bool = os.getenv("DEMO_MODE", "false").lower() in ("true", "1", "yes")
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings()