feat: add audit log table and integration with admin/tree endpoints

Creates AuditLog model with JSONB details column for tracking admin
actions. Integrates log_audit() helper into admin endpoints (role
change, team admin toggle, deactivate, activate) and tree delete.
IP address column reserved for future Railway proxy header support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/trees.py

@@ -14,6 +14,7 @@ from app.models.folder import UserFolder
 from app.schemas.tree import TreeCreate, TreeUpdate, TreeResponse, TreeListResponse, CategoryInfo
 from app.api.deps import get_current_active_user, require_engineer_or_admin, require_admin
 from app.core.permissions import can_edit_tree, can_access_tree
+from app.core.audit import log_audit
 
 router = APIRouter(prefix="/trees", tags=["trees"])
 
@@ -518,5 +519,7 @@ async def delete_tree(
         )
 
     tree.is_active = False
+    await log_audit(db, current_user.id, "tree.delete", "tree", tree.id,
+                    {"tree_name": tree.name})
     await db.commit()
     return None