perf: reduce memory usage by ~200MB+ at startup (#2013)

* perf: reduce memory usage by ~200MB+ at startup

Two key optimizations:

1. Use importlib.util.find_spec() instead of __import__() in dependency
   checking. find_spec() only locates modules without executing them,
   avoiding loading all 36 dependencies (~222MB) into memory at startup.

2. Introduce shared aiohttp.ClientSession via httpclient module.
   Previously, every HTTP request created a new ClientSession, which
   creates a new TCPConnector and SSL context, loading system root
   certificates each time (~270MB total allocations observed via memray).
   Now all HTTP client code reuses shared sessions.

   - satori.py and coze_server_api/client.py are left unchanged as they
     create one session per adapter lifecycle (not per-request).

Profiling data (memray):
- Peak memory: 403MB
- SSL context creation: 270MB / 6.7M allocations (67% of total)
- Dependency import: 222MB (55% of peak)
- Expected reduction: 150-350MB at startup

* fix: remove unused aiohttp imports (ruff F401)

* style: ruff format

src/langbot/pkg/core/bootutils/deps.py

@@ -1,3 +1,4 @@
+import importlib.util
 import pip
 import os
 from ...utils import pkgmgr
@@ -49,9 +50,10 @@ async def check_deps() -> list[str]:
 
     missing_deps = []
     for dep in required_deps:
-        try:
-            __import__(dep)
-        except ImportError:
+        # Use find_spec instead of __import__ to avoid actually loading
+        # all modules into memory. find_spec only checks if the module
+        # can be found, without executing module-level code.
+        if importlib.util.find_spec(dep) is None:
             missing_deps.append(dep)
     return missing_deps