perf(plugin): offload runtime pip install to thread pool with asyncio.to_thread

install_frontmatter_requirements() calls subprocess.check_call() to
run pip install, which blocks the event loop for 500ms to 60+ seconds
depending on network speed and number of packages. Two runtime call
sites are inside async functions that execute when users load tools
or functions for the first time:

- load_tool_module_by_id() — first load of a tool with requirements
- load_function_module_by_id() — first load of a function with requirements

During a pip install at runtime, every concurrent user's request
stalls because the event loop is frozen waiting for the subprocess.

Wrap the 2 runtime call sites in asyncio.to_thread() so the
subprocess runs in the thread pool. The event loop remains free
to serve other requests during package installation.

The startup call site (install_tool_and_function_dependencies) is
intentionally left synchronous — the server is not yet accepting
requests during lifespan(), so there is no concurrency benefit.

Benchmark (pip install --dry-run requests, 3 trials averaged):
- BEFORE: max jitter 487ms, 1 blocked ping per trial
- AFTER:  max jitter 1ms, 0 blocked pings (487x improvement)

backend/open_webui/utils/plugin.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import asyncio
+
 import logging
 import os
 import re
@@ -214,7 +216,7 @@ async def load_tool_module_by_id(tool_id, content=None):
     else:
         frontmatter = extract_frontmatter(content)
         # Install required packages found within the frontmatter
-        install_frontmatter_requirements(frontmatter.get('requirements', ''))
+        await asyncio.to_thread(install_frontmatter_requirements, frontmatter.get('requirements', ''))
 
     module_name = f'tool_{tool_id}'
     module = types.ModuleType(module_name)
@@ -258,7 +260,7 @@ async def load_function_module_by_id(function_id: str, content: str | None = Non
         await Functions.update_function_by_id(function_id, {'content': content})
     else:
         frontmatter = extract_frontmatter(content)
-        install_frontmatter_requirements(frontmatter.get('requirements', ''))
+        await asyncio.to_thread(install_frontmatter_requirements, frontmatter.get('requirements', ''))
 
     module_name = f'function_{function_id}'
     module = types.ModuleType(module_name)