feat: add web UI with login, CRUD, admin, and API key management

- Add login page with JWT authentication
- Add dashboard with stats and quick actions
- Add links management page (full CRUD with search)
- Add collections management page
- Add API key management page with copy-to-clipboard
- Add admin user management page (admin only)
- Fix UUID type mismatches across all endpoints
- Add updated_at column to api_keys and audit_log in schema.sql
- Fix DB_PASSWORD default in docker-compose.yml
- Add PyJWT to requirements.txt
- Fix API docs URL (/docs instead of /api/docs)
- Improve JS error handling (show actual messages)
- Rewrite conftest.py with proper DB lifecycle management
- Add 42 new integration tests (84 total, all passing)
  - test_admin.py: 15 tests for admin endpoints
  - test_auth_extended.py: 9 tests for API key CRUD
  - test_tags.py: 12 tests for tag endpoints
  - test_sync.py: 6 tests for sync endpoints

LinkSyncServer/templates/admin.html

@@ -0,0 +1,72 @@
+{% extends "base.html" %}
+
+{% block title %}Admin - LinkSync{% endblock %}
+
+{% block content %}
+<div class="page-header">
+    <h1>User Management</h1>
+    <button class="btn btn-primary" id="new-user-btn">+ New User</button>
+</div>
+
+<div id="users-list" class="users-table">
+    <div class="loading">Loading users...</div>
+</div>
+
+<div id="user-modal" class="modal" style="display: none;">
+    <div class="modal-overlay"></div>
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 id="user-modal-title">Create User</h2>
+            <button class="modal-close" id="user-modal-close">&times;</button>
+        </div>
+        <form id="user-form">
+            <input type="hidden" id="user-id">
+            <div class="form-group">
+                <label for="user-username">Username *</label>
+                <input type="text" id="user-username" required>
+            </div>
+            <div class="form-group">
+                <label for="user-email">Email *</label>
+                <input type="email" id="user-email" required>
+            </div>
+            <div class="form-group">
+                <label for="user-password">Password *</label>
+                <input type="password" id="user-password" required>
+            </div>
+            <div class="form-group">
+                <label for="user-role">Role</label>
+                <select id="user-role">
+                    <option value="user">User</option>
+                    <option value="admin">Admin</option>
+                </select>
+            </div>
+            <div class="form-group checkbox-group">
+                <label>
+                    <input type="checkbox" id="user-active" checked>
+                    Active
+                </label>
+            </div>
+            <div class="form-actions">
+                <button type="button" class="btn btn-secondary" id="user-cancel-btn">Cancel</button>
+                <button type="submit" class="btn btn-primary" id="user-save-btn">Save</button>
+            </div>
+        </form>
+    </div>
+</div>
+
+<div id="delete-user-modal" class="modal" style="display: none;">
+    <div class="modal-overlay"></div>
+    <div class="modal-content modal-sm">
+        <h2>Delete User</h2>
+        <p>Are you sure you want to delete this user? This action cannot be undone.</p>
+        <div class="form-actions">
+            <button class="btn btn-secondary" id="delete-user-cancel-btn">Cancel</button>
+            <button class="btn btn-danger" id="confirm-delete-user-btn">Delete</button>
+        </div>
+    </div>
+</div>
+{% endblock %}
+
+{% block extra_js %}
+<script src="/static/js/admin-page.js"></script>
+{% endblock %}