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>

src/dashboard/middleware/csrf.py

@@ -7,7 +7,6 @@ to protect state-changing endpoints from cross-site request attacks.
 import secrets
 import hmac
 import hashlib
-import os
 from typing import Callable, Optional
 from functools import wraps
 
@@ -125,7 +124,8 @@ class CSRFMiddleware(BaseHTTPMiddleware):
         For unsafe methods: Validate the CSRF token.
         """
         # Bypass CSRF if explicitly disabled (e.g. in tests)
-        if os.environ.get("TIMMY_DISABLE_CSRF") == "1":
+        from config import settings
+        if settings.timmy_disable_csrf:
             return await call_next(request)
 
         # Get existing CSRF token from cookie
@@ -142,7 +142,7 @@ class CSRFMiddleware(BaseHTTPMiddleware):
                     key=self.cookie_name,
                     value=new_token,
                     httponly=False,  # Must be readable by JavaScript
-                    secure=False,    # Set to True in production with HTTPS
+                    secure=settings.csrf_cookie_secure,
                     samesite="Lax",
                     max_age=86400    # 24 hours
                 )