feat: add app-level authentication with login, session cookies, and admin panel

Replace Nginx basic auth with proper user accounts:
- Session cookies via itsdangerous (30-day expiry, httponly, secure)
- Password hashing with bcrypt via passlib
- Login page with AC-themed UI
- Admin page for user management (CRUD)
- AuthMiddleware exempts plugin WS and browser WS endpoints
- Issues/comments author auto-populated from session
- Sidebar shows logged-in username, admin link, and logout
- Seed users: erik (admin), alex, lundberg
- SECRET_KEY env var for cookie signing

static/style.css

@@ -117,6 +117,8 @@ body {
   box-sizing: border-box;
   padding: 18px 16px;
   overflow-y: auto;
+  display: flex;
+  flex-direction: column;
 }
 #sidebar h2 {
   margin: 8px 0 12px;
@@ -2832,3 +2834,41 @@ table.ts-allegiance td:first-child {
 .issue-comment-form {
   margin-top: 4px;
 }
+
+/* ---------- User info section (sidebar bottom) ---------- */
+.user-info {
+  margin-top: auto;
+  padding: 10px 0 0;
+  border-top: 1px solid #333;
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 0.75rem;
+  flex-shrink: 0;
+}
+
+.user-info-name {
+  color: #d4af37;
+  font-weight: bold;
+}
+
+.user-info-admin {
+  color: #8a7a44;
+  text-decoration: none;
+  font-size: 0.7rem;
+}
+
+.user-info-admin:hover {
+  color: #d4af37;
+}
+
+.user-info-logout {
+  margin-left: auto;
+  color: #888;
+  text-decoration: none;
+  font-size: 0.7rem;
+}
+
+.user-info-logout:hover {
+  color: #ff6b6b;
+}