feat(settings): add log retention settings page

Sysadmin/admin-only settings page for configuring how long device log
history is kept before pruning. GET/PUT /api/settings/log-retention,
new LogRetentionSettings page, nav entry, and route.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/settings/log_retention_models.py

@@ -0,0 +1,10 @@
+from pydantic import BaseModel
+from typing import Optional
+
+
+class LogRetentionSettings(BaseModel):
+    days: int = 180
+
+
+class LogRetentionSettingsUpdate(BaseModel):
+    days: Optional[int] = None

backend/settings/log_retention_service.py

@@ -0,0 +1,31 @@
+from shared.firebase import get_db
+from settings.log_retention_models import LogRetentionSettings, LogRetentionSettingsUpdate
+
+COLLECTION = "admin_settings"
+DOC_ID = "log_retention"
+
+
+def get_log_retention() -> LogRetentionSettings:
+    """Get log retention settings from Firestore. Creates defaults if not found."""
+    db = get_db()
+    doc = db.collection(COLLECTION).document(DOC_ID).get()
+    if doc.exists:
+        return LogRetentionSettings(**doc.to_dict())
+    defaults = LogRetentionSettings()
+    db.collection(COLLECTION).document(DOC_ID).set(defaults.model_dump())
+    return defaults
+
+
+def update_log_retention(data: LogRetentionSettingsUpdate) -> LogRetentionSettings:
+    """Update log retention settings. Only provided fields are updated."""
+    db = get_db()
+    doc_ref = db.collection(COLLECTION).document(DOC_ID)
+    doc = doc_ref.get()
+
+    existing = doc.to_dict() if doc.exists else LogRetentionSettings().model_dump()
+    update_data = data.model_dump(exclude_none=True)
+    existing.update(update_data)
+
+    normalized = LogRetentionSettings(**existing)
+    doc_ref.set(normalized.model_dump())
+    return normalized

backend/settings/router.py

@@ -4,8 +4,10 @@ from auth.dependencies import require_permission, require_roles
 from auth.models import Role
 from settings.models import MelodySettings, MelodySettingsUpdate
 from settings.public_features_models import PublicFeaturesSettings, PublicFeaturesSettingsUpdate
+from settings.log_retention_models import LogRetentionSettings, LogRetentionSettingsUpdate
 from settings import service
 from settings import public_features_service
+from settings import log_retention_service
 
 router = APIRouter(prefix="/api/settings", tags=["settings"])
 
@@ -40,3 +42,20 @@ async def update_public_features(
     _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
 ):
     return public_features_service.update_public_features(body)
+
+
+# ── Log Retention Settings (sysadmin / admin only) ───────────────────────────
+
+@router.get("/log-retention", response_model=LogRetentionSettings)
+async def get_log_retention(
+    _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
+):
+    return log_retention_service.get_log_retention()
+
+
+@router.put("/log-retention", response_model=LogRetentionSettings)
+async def update_log_retention(
+    body: LogRetentionSettingsUpdate,
+    _user: TokenPayload = Depends(require_roles(Role.sysadmin, Role.admin)),
+):
+    return log_retention_service.update_log_retention(body)