# Action Type Field Update

## Context
The `action_type` field was previously defined as a `SMALLINT` with integer mappings (1, 2, 3, 4). The requirement has changed to store explicit string values for better readability and direct mapping.

## Changes
1.  **Field Definition**:
    *   Changed `action_type` from `SMALLINT` to `VARCHAR(20)`.

2.  **Enum Values**:
    *   Old: `1` (ACK), `2` (Control), `4` (Status)
    *   New:
        *   `"0FACK"`: ACK (0x0F 上报)
        *   `"0F下发"`: Control (0x0F 下发)
        *   `"36上报"`: Status (0x36)

3.  **Document Updates**:
    *   `docs/readme.md`: Updated table definition and dictionary.
    *   `docs/kafka_format.md`: Updated JSON schema and backend logic examples.

4.  **Code Updates**:
    *   `src/processor/index.js`: Updated `resolveActionType` to return the new string enums.
