fix: switch Big Pickle -> DeepSeek V4 Flash (censorship)

- Big Pickle (opencode/zen free) censura contenuti fantasy
  con "User Safety: safe" - blocca la narrazione del DM
- DeepSeek V4 Flash su opencode/go: $0.14/M token,
  nessuna censura, descrizioni fantasy corrette
- Aggiornati .env, start.sh, config.py

src/engine/config.py

@@ -15,9 +15,9 @@ class ProviderConfig:
     openrouter_model: str = "openrouter/free"
     openrouter_key_env: str = "DCC_OPENROUTER_KEY"
     openrouter_free_fallback: bool = True
-    local_base_url: str = "https://opencode.ai/zen/v1"
+    local_base_url: str = "https://opencode.ai/zen/go/v1"
     local_api_key: str = ""
-    local_model: str = "big-pickle"
+    local_model: str = "deepseek-v4-flash"
     timeout: int = 120