Fix remaining code quality and infrastructure items

- admin.py: remove unused get_current_user import
- feed.py, flock.py, other.py: add IntegrityError handling on POST/PUT
  endpoints; duplicate submissions now return 409 instead of crashing with
  a 500 error
- stats.py: extract magic numbers into named module-level constants
  (DAYS_ROLLING, DAYS_SHORT, PRECISION_AVG, PRECISION_HEN, PRECISION_COST);
  add return type annotations to _total_feed_cost and _total_other_cost;
  normalize both helpers to always return Decimal so budget_stats no longer
  needs Decimal(str(...)) workarounds; simplify _cpe/_cpd helpers
- dashboard.js: read --green CSS variable at runtime instead of hardcoding
  the hex value so chart color stays in sync with the stylesheet
- docker-compose.yml: add healthcheck to api service (polls /api/health
  every 30s) so Docker knows when the API is unhealthy; add password
  strength guidance comment above the db service

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

backend/routers/admin.py

@@ -7,7 +7,7 @@ from sqlalchemy.orm import Session
 from database import get_db
 from models import User
 from schemas import UserCreate, UserOut, ResetPasswordRequest, TokenResponse
-from auth import hash_password, create_access_token, get_current_admin, get_current_user
+from auth import hash_password, create_access_token, get_current_admin
 
 router = APIRouter(prefix="/api/admin", tags=["admin"])
 logger = logging.getLogger("yolkbook")