docs: add API documentation

Co-authored-by: aider (gemini/gemini-2.5-pro-preview-05-06) <aider@aider.chat>

docs/api.md

@@ -0,0 +1,90 @@
+# NTP Timeturner API
+
+This document describes the HTTP API for the NTP Timeturner application.
+
+## Endpoints
+
+### Status
+
+- **`GET /api/status`**
+
+  Retrieves the real-time status of the LTC reader and system clock synchronization.
+
+  **Example Response:**
+  ```json
+  {
+    "ltc_status": "LOCK",
+    "ltc_timecode": "10:20:30:00",
+    "frame_rate": "25.00fps",
+    "system_clock": "10:20:30.005",
+    "timecode_delta_ms": 5,
+    "timecode_delta_frames": 0,
+    "sync_status": "IN SYNC",
+    "jitter_status": "GOOD",
+    "lock_ratio": 99.5,
+    "ntp_active": true,
+    "interfaces": ["192.168.1.100"],
+    "hardware_offset_ms": 0
+  }
+  ```
+
+### Sync
+
+- **`POST /api/sync`**
+
+  Triggers a manual synchronization of the system clock to the current LTC timecode. This requires the application to have `sudo` privileges to execute the `date` command.
+
+  **Request Body:** None
+
+  **Success Response:**
+  ```json
+  {
+    "status": "success",
+    "message": "Sync command issued."
+  }
+  ```
+
+  **Error Responses:**
+  ```json
+  {
+    "status": "error",
+    "message": "No LTC timecode available to sync to."
+  }
+  ```
+  ```json
+  {
+    "status": "error",
+    "message": "Sync command failed."
+  }
+  ```
+
+### Configuration
+
+- **`GET /api/config`**
+
+  Retrieves the current application configuration.
+
+  **Example Response:**
+  ```json
+  {
+    "hardware_offset_ms": 0
+  }
+  ```
+
+- **`POST /api/config`**
+
+  Updates the `hardware_offset_ms` configuration. The new value is persisted to `config.json` and reloaded by the application automatically.
+
+  **Example Request:**
+  ```json
+  {
+    "hardware_offset_ms": 10
+  }
+  ```
+
+  **Success Response:**
+  ```json
+  {
+    "hardware_offset_ms": 10
+  }
+  ```