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/other.py

@@ -2,6 +2,7 @@ from datetime import date
 from typing import Optional
 from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from database import get_db
@@ -39,7 +40,11 @@ def create_other_purchase(
 ):
     record = OtherPurchase(**body.model_dump(), user_id=current_user.id)
     db.add(record)
-    db.commit()
+    try:
+        db.commit()
+    except IntegrityError:
+        db.rollback()
+        raise HTTPException(status_code=409, detail=f"An other purchase for {body.date} already exists.")
     db.refresh(record)
     return record
 
@@ -59,7 +64,11 @@ def update_other_purchase(
         raise HTTPException(status_code=404, detail="Record not found")
     for field, value in body.model_dump(exclude_none=True).items():
         setattr(record, field, value)
-    db.commit()
+    try:
+        db.commit()
+    except IntegrityError:
+        db.rollback()
+        raise HTTPException(status_code=409, detail="An other purchase for that date already exists.")
     db.refresh(record)
     return record