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>

nginx/html/js/dashboard.js

@@ -35,13 +35,15 @@ function buildChart(eggs) {
 
     if (eggChart) eggChart.destroy();   // prevent duplicate charts on re-render
 
+    const green = getComputedStyle(document.documentElement).getPropertyValue('--green').trim();
+
     eggChart = new Chart(ctx, {
         type: 'line',
         data: {
             labels,
             datasets: [{
                 data,
-                borderColor:     '#3d6b4f',
+                borderColor:     green,
                 backgroundColor: 'rgba(61,107,79,0.08)',
                 borderWidth: 2,
                 pointRadius: 3,