Security hardening: secrets, validation, Docker, and error handling

- Add root .gitignore to prevent btc_wallet.py (with RPC credentials) from being committed
- Load JWT SECRET_KEY from environment variable instead of hardcoded value
- Restrict CORS to explicit methods/headers instead of wildcards
- Add Pydantic Field validation (gt=0) to purchase amounts and user credentials
- Add logging to all silent exception handlers in btc.py
- Run backend and frontend Docker containers as non-root appuser
- Add .dockerignore for both backend and frontend
- Pass SECRET_KEY env var through docker-compose; add healthchecks to both services
- Update bcrypt from pinned 3.2.2 to >=4.0.0
- Capture error objects in frontend catch blocks; check admin delete response

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/routes/users.py

@@ -1,6 +1,6 @@
 from fastapi import APIRouter, Depends, HTTPException, status
 from sqlalchemy.orm import Session
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from ..database import get_db
 from .. import models
@@ -10,8 +10,8 @@ router = APIRouter()
 
 
 class UserCreate(BaseModel):
-    username: str
-    password: str
+    username: str = Field(min_length=3, max_length=50)
+    password: str = Field(min_length=8)
 
 
 class Token(BaseModel):