refactor: deep review fixes round 3

- client.py:
  - Remove dead _extract_cursor second branch (unreachable code)
  - Cache SSL context as module-level _SSL_CTX (avoid re-reading CA certs)
  - Add 404 stale-fallback retry to _graphql_post (parity with GET)
  - Remove dead core.get('name')/core.get('screen_name') in fetch_user
  - Set Content-Type: application/json only for POST requests
  - Rename _to_int → _parse_int for clarity vs config._as_int
  - Add 'not thread-safe' note on module-level caches

- cli.py:
  - _fetch_and_display now accepts optional config param (fix double load)
  - Refactor user_posts to use _fetch_and_display
  - Pass config to all _fetch_and_display callers

- pyproject.toml:
  - Move xclienttransaction/requests to optional [transaction] deps
  - Add beautifulsoup4 to [transaction] optional deps

- README.md:
  - Add rateLimit config section with comments
  - Add constants.py to project structure tree

pyproject.toml

@@ -30,11 +30,14 @@ dependencies = [
     "click>=8.0",
     "rich>=13.0",
     "PyYAML>=6.0",
-    "xclienttransaction>=1.0.1",
-    "requests>=2.32.4",
 ]
 
 [project.optional-dependencies]
+transaction = [
+    "xclienttransaction>=1.0.1",
+    "requests>=2.32.4",
+    "beautifulsoup4>=4.12",
+]
 dev = [
     "pytest>=8.0",
     "ruff>=0.8",