feat(04-01): UnsplashService + Settings unsplash_api_key

- Crea UnsplashService con search, cache disco, traduzione IT->EN
- ~30 keyword B2B italiane tradotte in dizionario statico
- Cache in-memory + persistenza su disco (unsplash_cache.json)
- Retry automatico su errori di rete, no-retry su 401/403
- Rate limiting awareness via X-Ratelimit-Remaining header
- Aggiunge campo unsplash_api_key a Settings schema
- Router settings espone unsplash_api_key_masked + configured
- Merge None-preserving per unsplash_api_key nel PUT

backend/routers/settings.py

@@ -35,6 +35,7 @@ class SettingsStatusResponse(BaseModel):
     """Risposta per GET /status — usata dal frontend per abilitare/disabilitare il pulsante genera."""
     api_key_configured: bool
     llm_model: str
+    unsplash_api_key_configured: bool
 
 
 class SettingsResponse(BaseModel):
@@ -46,6 +47,7 @@ class SettingsResponse(BaseModel):
     frequenza_post: int
     brand_name: Optional[str]
     tono: Optional[str]
+    unsplash_api_key_masked: Optional[str]  # Solo ultimi 4 caratteri o None
 
 
 # ---------------------------------------------------------------------------
@@ -103,6 +105,7 @@ async def get_settings_status() -> SettingsStatusResponse:
     return SettingsStatusResponse(
         api_key_configured=bool(settings.api_key),
         llm_model=settings.llm_model,
+        unsplash_api_key_configured=bool(settings.unsplash_api_key),
     )
 
 
@@ -125,6 +128,7 @@ async def get_settings() -> SettingsResponse:
         frequenza_post=settings.frequenza_post,
         brand_name=settings.brand_name,
         tono=settings.tono,
+        unsplash_api_key_masked=_mask_api_key(settings.unsplash_api_key),
     )
 
 
@@ -149,8 +153,12 @@ async def update_settings(new_settings: Settings) -> SettingsResponse:
     if new_settings.api_key is None:
         new_settings = new_settings.model_copy(update={"api_key": existing.api_key})
 
+    # Se la nuova unsplash_api_key è None, mantieni quella esistente (stessa logica)
+    if new_settings.unsplash_api_key is None:
+        new_settings = new_settings.model_copy(update={"unsplash_api_key": existing.unsplash_api_key})
+
     _save_settings(new_settings)
-    logger.info("Settings aggiornate | model=%s | brand=%s", new_settings.llm_model, new_settings.brand_name)
+    logger.info("Settings aggiornate | model=%s | brand=%s | unsplash=%s", new_settings.llm_model, new_settings.brand_name, bool(new_settings.unsplash_api_key))
 
     return SettingsResponse(
         api_key_masked=_mask_api_key(new_settings.api_key),
@@ -160,4 +168,5 @@ async def update_settings(new_settings: Settings) -> SettingsResponse:
         frequenza_post=new_settings.frequenza_post,
         brand_name=new_settings.brand_name,
         tono=new_settings.tono,
+        unsplash_api_key_masked=_mask_api_key(new_settings.unsplash_api_key),
     )