refactor: centralize config & harden security (#141)

* feat: upgrade primary model from llama3.1:8b to qwen2.5:14b

- Swap OLLAMA_MODEL_PRIMARY to qwen2.5:14b for better reasoning
- llama3.1:8b-instruct becomes fallback
- Update .env default and README quick start
- Fix hardcoded model assertions in tests

qwen2.5:14b provides significantly better multi-step reasoning
and tool calling reliability while still running locally on
modest hardware. The 8B model remains as automatic fallback.

* security: centralize config, harden uploads, fix silent exceptions

- Add 9 pydantic Settings fields (skip_embeddings, disable_csrf,
  rqlite_url, brain_source, brain_db_path, csrf_cookie_secure,
  chat_api_max_body_bytes, timmy_test_mode) to centralize env-var access
- Migrate 8 os.environ.get() calls across 5 source files to use
  `from config import settings` per project convention
- Add path traversal defense-in-depth to file upload endpoint
- Add 1MB request body size limit to chat API
- Make CSRF cookie secure flag configurable via settings
- Replace 2 silent `except: pass` blocks with debug logging in session.py
- Remove unused `import os` from brain/memory.py and csrf.py
- Update 5 CSRF test fixtures to patch settings instead of os.environ

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

---------

Co-authored-by: Trip T <trip@local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/dashboard/middleware/test_csrf.py

@@ -12,14 +12,11 @@ class TestCSRFMiddleware:
     @pytest.fixture(autouse=True)
     def enable_csrf(self):
         """Re-enable CSRF for these tests."""
-        import os
-        old_val = os.environ.get("TIMMY_DISABLE_CSRF")
-        os.environ["TIMMY_DISABLE_CSRF"] = "0"
+        from config import settings
+        original = settings.timmy_disable_csrf
+        settings.timmy_disable_csrf = False
         yield
-        if old_val is not None:
-            os.environ["TIMMY_DISABLE_CSRF"] = old_val
-        else:
-            del os.environ["TIMMY_DISABLE_CSRF"]
+        settings.timmy_disable_csrf = original
 
     def test_csrf_token_generation(self):
         """CSRF token should be generated and stored in session/state."""