Add transparent PyPI proxy and improve upstream sources UI

- Implement PEP 503 Simple API endpoints for pip compatibility:
  - GET /pypi/simple/ - package index
  - GET /pypi/simple/{package}/ - version list with rewritten links
  - GET /pypi/simple/{package}/{filename} - download with auto-caching

- Improve upstream sources table UI:
  - Center text under column headers
  - Remove separate Source column, show ENV badge inline with name
  - Make Test/Edit buttons more prominent with secondary button style

frontend/src/pages/AdminCachePage.css

@@ -65,7 +65,7 @@
 .sources-table th,
 .sources-table td {
   padding: 0.75rem 1rem;
-  text-align: left;
+  text-align: center;
   border-bottom: 1px solid var(--border-color);
 }
 
@@ -91,6 +91,11 @@
   white-space: nowrap;
 }
 
+/* Name column should be left-aligned */
+.sources-table td:first-child {
+  text-align: left;
+}
+
 .url-cell {
   font-family: monospace;
   font-size: 0.9rem;
@@ -98,6 +103,7 @@
   overflow: hidden;
   text-overflow: ellipsis;
   white-space: nowrap;
+  text-align: left;
 }
 
 /* Badges */
@@ -243,10 +249,22 @@
 }
 
 .btn-sm {
-  padding: 0.25rem 0.5rem;
+  padding: 0.25rem 0.75rem;
   font-size: 0.8rem;
 }
 
+.btn-secondary {
+  background-color: var(--bg-tertiary);
+  border-color: var(--border-color);
+  color: var(--text-primary);
+  font-weight: 500;
+}
+
+.btn-secondary:hover {
+  background-color: var(--bg-secondary);
+  border-color: var(--text-secondary);
+}
+
 .empty-message {
   color: var(--text-secondary);
   font-style: italic;

frontend/src/pages/AdminCachePage.tsx

@@ -272,8 +272,7 @@ function AdminCachePage() {
                 <th>URL</th>
                 <th>Priority</th>
                 <th>Status</th>
-                <th>Source</th>
-                <th></th>
+                <th>Test</th>
                 <th>Actions</th>
               </tr>
             </thead>
@@ -282,24 +281,18 @@ function AdminCachePage() {
                 <tr key={source.id} className={source.enabled ? '' : 'disabled-row'}>
                   <td>
                     <span className="source-name">{source.name}</span>
+                    {source.source === 'env' && (
+                      <span className="env-badge" title="Defined via environment variable">ENV</span>
+                    )}
                   </td>
                   <td>{source.source_type}</td>
-                  <td className="url-cell">{source.url}</td>
+                  <td className="url-cell" title={source.url}>{source.url}</td>
                   <td>{source.priority}</td>
                   <td>
                     <span className={`status-badge ${source.enabled ? 'enabled' : 'disabled'}`}>
                       {source.enabled ? 'Enabled' : 'Disabled'}
                     </span>
                   </td>
-                  <td>
-                    {source.source === 'env' ? (
-                      <span className="env-badge" title="Defined via environment variable">
-                        ENV
-                      </span>
-                    ) : (
-                      'Database'
-                    )}
-                  </td>
                   <td className="test-cell">
                     {testingId === source.id ? (
                       <span className="test-dot testing" title="Testing...">●</span>
@@ -317,14 +310,14 @@ function AdminCachePage() {
                   </td>
                   <td className="actions-cell">
                     <button
-                      className="btn btn-sm"
+                      className="btn btn-sm btn-secondary"
                       onClick={() => handleTest(source)}
                       disabled={testingId === source.id}
                     >
                       Test
                     </button>
                     {source.source !== 'env' && (
-                      <button className="btn btn-sm" onClick={() => openEditForm(source)}>
+                      <button className="btn btn-sm btn-secondary" onClick={() => openEditForm(source)}>
                         Edit
                       </button>
                     )}