Refactor V3 CRUD: Extract helper functions and unify sanitization logic.

- Created app/lib_api_crud_v3.py to house core security, filtering, and sanitization logic.
- Implemented reusable sanitize_payload() to generically strip virtual lookup fields (*_id_random) and view-only fields (fields_to_exclude_from_db).
- Updated app/routers/api_crud_v3.py to use the new library and consolidated sanitization across all Create/Update endpoints.
- Documented Phase 1 completion in documentation/REFACTOR_API_CRUD_V3.md.

documentation/REFACTOR_API_CRUD_V3.md

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+# Refactoring Plan: API CRUD V3
+
+**Goal:** Modularize `app/routers/api_crud_v3.py` to improve maintainability, readability, and reusability. The file currently mixes route definitions, security enforcement, data sanitization, and helper utilities.
+
+## Phase 1: Extract Helpers & Core Logic (Safest) - COMPLETED
+**Objective:** Move pure functions and business logic out of the router file.
+
+1.  **Create `app/lib_api_crud_v3.py`**: DONE
+2.  **Update `app/routers/api_crud_v3.py`**: DONE (All endpoints now use `sanitize_payload`).
+
+## Phase 2: Separate Child/Nested Routes - PLANNED
+
+1.  **Create `app/routers/api_crud_v3_nested.py`**:
+    *   Move `get_child_obj_li`
+    *   Move `post_child_obj`
+    *   Move `get_child_obj`
+    *   Move `patch_child_obj`
+    *   Move `delete_child_obj`
+
+2.  **Update `app/main.py` (or router inclusion)**:
+    *   Ensure the new router is included, OR include it within `api_crud_v3.py` if preferred to keep a single import point.
+
+## Phase 3: Schema Introspection
+**Objective:** Isolate database introspection logic.
+
+1.  **Create `app/lib_schema_v3.py` (or similar)**:
+    *   Move the logic inside `get_obj_schema` (SQL `DESCRIBE` parsing, Pydantic introspection) to a helper function.
+
+## Execution Strategy
+We will execute **Phase 1** first as it provides immediate value (removing code duplication for sanitization) with minimal risk to routing logic.
+
+### Testing
+After each move:
+1.  Run `tests/test_v3_router_filtering.py` (requires update to import from new location if we test the lib directly).
+2.  Verify application startup.