fix(deps+tests): move httpx to prod deps; stub Poller+Scheduler in sync test

httpx was in dev deps only, causing ImportError for users doing `pip install -e .`
since atm.commands imports httpx at module level. Moved to main dependencies.

Also stubs TelegramPoller and ScreenshotScheduler in the sync catchup test to
prevent flaky CI failures from attempted real network connections.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -13,6 +13,7 @@ dependencies = [
     "pillow>=10.0",
     "requests>=2.31",
     "rich>=13.0",
+    "httpx>=0.27",
 ]
 
 [project.optional-dependencies]
@@ -25,7 +26,6 @@ dev = [
     "pytest>=8.0",
     "pytest-cov>=5.0",
     "pytest-asyncio>=0.23",
-    "httpx>=0.27",
 ]
 
 [project.scripts]