feat: add favicon.ico and gate model write APIs by env flag

- Generate and serve real /favicon.ico from static assets
- Update HTML to use /favicon.ico
- Add ENABLE_MODEL_RW_API setting (default: false)
- Disable POST/DELETE model endpoints by default
- Hide write endpoints from OpenAPI when disabled
- Return 404 for write endpoints when disabled
- Update env.example with ENABLE_MODEL_RW_API documentation
- Update README and PRD with R/W API policy and remote compose notes
- Add tests to verify write endpoints are disabled by default

app/api/models.py

@@ -13,6 +13,15 @@ from app.config import settings
 logger = logging.getLogger(__name__)
 router = APIRouter()
 
+
+def ensure_rw_api_enabled() -> None:
+    """Blocca le API di scrittura se non abilitate esplicitamente."""
+    if not settings.ENABLE_MODEL_RW_API:
+        raise HTTPException(
+            status_code=404,
+            detail="Endpoint non disponibile"
+        )
+
 class ModelInfo(BaseModel):
     """Informazioni su un modello"""
     name: str
@@ -165,7 +174,10 @@ async def get_model(model_name: str):
             detail="Errore nel recupero del modello"
         )
 
-@router.post("/models/{model_name}/pull")
+@router.post(
+    "/models/{model_name}/pull",
+    include_in_schema=settings.ENABLE_MODEL_RW_API
+)
 async def pull_model(model_name: str):
     """
     Scarica/carica un modello in Ollama
@@ -176,6 +188,7 @@ async def pull_model(model_name: str):
     Returns:
         dict: Status del download
     """
+    ensure_rw_api_enabled()
     try:
         response = requests.post(
             f"{settings.OLLAMA_HOST}/api/pull",
@@ -198,7 +211,10 @@ async def pull_model(model_name: str):
             detail="Errore nel pull del modello"
         )
 
-@router.delete("/models/{model_name}")
+@router.delete(
+    "/models/{model_name}",
+    include_in_schema=settings.ENABLE_MODEL_RW_API
+)
 async def delete_model(model_name: str):
     """
     Elimina un modello da Ollama
@@ -209,6 +225,7 @@ async def delete_model(model_name: str):
     Returns:
         dict: Confirmazione eliminazione
     """
+    ensure_rw_api_enabled()
     try:
         response = requests.delete(
             f"{settings.OLLAMA_HOST}/api/delete",