Add preview guide for ambassador OTC desk

ambassadors/fastapi-ambassador-leaderboard/OTC_PREVIEW.md

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+# Ambassador OTC Desk Preview Guide
+
+This guide captures the environment variables and runtime steps used to preview the ambassador OTC desk locally.
+
+## Prerequisites
+
+* Python 3.12 with `venv`
+* Redis server running on `localhost:6379`
+
+Install Python dependencies into a virtual environment:
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+pip install --upgrade pip
+pip install fastapi uvicorn aiofiles jinja2 python-dotenv redis pandas httpx requests python-multipart
+```
+
+Start Redis (if it is not already running):
+
+```bash
+redis-server --daemonize yes
+```
+
+## Launching the FastAPI app
+
+Set the CSV paths so the leaderboard data loads correctly before starting Uvicorn:
+
+```bash
+export CSV_PATH=ambassadors/fastapi-ambassador-leaderboard/data/leaderboard.csv
+export REGISTRATIONS_CSV=ambassadors/fastapi-ambassador-leaderboard/data/registrations.csv
+uvicorn ambassadors.fastapi-ambassador-leaderboard.main:app --host 0.0.0.0 --port 8000 --reload
+```
+
+## Creating a test ambassador
+
+With Redis running, seed a verified ambassador account so you can sign in and access `/otc`:
+
+```python
+from redis import Redis
+import hashlib
+client = Redis.from_url('redis://localhost:6379/0', decode_responses=True)
+email = '[email protected]'
+password = 'password123'
+client.hset(f'user:{email}', mapping={
+    'password': hashlib.sha256(password.encode()).hexdigest(),
+    'verified': 1,
+    'telegram': '@previewer',
+    'wallet': 'itc1previewwallet'
+})
+client.hset('score_pad', email, 1000)
+```
+
+After logging in at `http://localhost:8000/login` with the seeded credentials, open the OTC desk at `http://localhost:8000/otc` to view the trading interface.