Add MCP (Model Context Protocol) server endpoint

- Implement MCP server module (aisbf/mcp.py) with SSE and HTTP stream support
- Add two authentication levels: autoselect-only and fullconfig
- Add MCP configuration to config/aisbf.json (disabled by default)
- Add MCP endpoints to main.py: GET/POST /mcp, GET /mcp/tools, POST /mcp/tools/call
- Both token types give access to standard APIs (list_models, chat_completion)
- autoselect_tokens: access to autoselection/autorotation settings
- fullconfig_tokens: access to full system configuration including providers

This allows remote agents to configure the system and/or just the
autoselections/rotations, as well as make requests for models.

config/aisbf.json

@@ -13,6 +13,17 @@
       "your-secret-token-2"
     ]
   },
+  "mcp": {
+    "enabled": false,
+    "autoselect_tokens": [
+      "mcp-autoselect-token-1",
+      "mcp-autoselect-token-2"
+    ],
+    "fullconfig_tokens": [
+      "mcp-fullconfig-token-1",
+      "mcp-fullconfig-token-2"
+    ]
+  },
   "dashboard": {
     "enabled": true,
     "username": "admin",