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/login.html

@@ -42,7 +42,7 @@
                 </div>
                 <div class="form-group" style="margin-bottom:1rem">
                     <label for="reg-password">Password</label>
-                    <input type="password" id="reg-password" autocomplete="new-password" required minlength="6" placeholder="min 6 characters">
+                    <input type="password" id="reg-password" autocomplete="new-password" required minlength="10" placeholder="min 10 characters">
                 </div>
                 <div class="form-group" style="margin-bottom:1.5rem">
                     <label for="reg-confirm">Confirm Password</label>
@@ -114,6 +114,7 @@
                     body: JSON.stringify({ username, password }),
                 });
                 const data = await res.json();
+                if (res.status === 429) { showError('login-msg', 'Too many attempts — please wait a minute and try again.'); return; }
                 if (!res.ok) { showError('login-msg', data.detail || 'Login failed'); return; }
                 localStorage.setItem('token', data.access_token);
                 window.location.href = '/';