Fix bugs, data integrity, and cache busting

- models.py: add UniqueConstraint(user_id, date) to flock_history so
  duplicate flock entries for the same day are rejected at the DB level
- main.py: v2.3 migration applies the new unique constraint to existing
  installs at startup
- login.html: update register form minlength and placeholder from 6 to 10
  characters to match backend; add specific 429 error message so rate-
  limited users see "Too many attempts — please wait a minute" instead of
  a generic failure
- auth.js: update settings modal password input minlength from 6 to 10
- summary.js: fix CSV export truncation — pass limit=10000 so users with
  more than 500 days of data get a complete export; read chart border color
  from --green CSS variable instead of hardcoded hex
- All HTML files: bump JS version params to ?v=4 so browsers discard
  cached copies of files changed across recent sessions (api.js, auth.js,
  dashboard.js, history.js, log.js, flock.js, budget.js, summary.js,
  admin.js)
- .env.example: add password strength guidance for MySQL and admin vars

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

nginx/html/index.html

@@ -68,12 +68,11 @@
             </table>
         </div>
 
-        <p class="text-muted mt-1"><a href="/history">View full history →</a></p>
     </main>
 
     <script src="https://cdn.jsdelivr.net/npm/chart.js@4.4.0/dist/chart.umd.min.js"></script>
-    <script src="/js/api.js?v=3"></script>
-    <script src="/js/auth.js?v=3"></script>
-    <script src="/js/dashboard.js?v=2"></script>
+    <script src="/js/api.js?v=4"></script>
+    <script src="/js/auth.js?v=4"></script>
+    <script src="/js/dashboard.js?v=4"></script>
 </body>
 </html>