docs: add backend API documentation

docs/backend/backend_python/api.md

@@ -1,7 +1,41 @@
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 <!-- # API Reference -->
+## Backend API Architecture
 
+PictoPy's Python backend exposes its HTTP API through FastAPI. Requests are routed from the application entry point to the corresponding route module, which interacts with the database and utility layers before returning a Pydantic response model.
+
+```mermaid
+flowchart TD
+    A[Client] -->|HTTP Request| B[FastAPI Application]
+    B --> C[API Router]
+    C --> D[Route Handler]
+    D --> E[Database Layer]
+    D --> F[Utility Layer]
+    E --> G[(SQLite Database)]
+    D --> H[Pydantic Response Model]
+    H -->|JSON Response| A
+```
+## Image Retrieval API Flow
+
+The `GET /images/` endpoint provides a concrete example of the backend request/response flow. The request is routed to `get_all_images()`, which retrieves image records through `db_get_all_images()`. The database layer retrieves image and tag data from SQLite and parses stored metadata. `get_all_images()` invokes `image_util_parse_metadata()` again while constructing each `ImageData` item before returning the response model.
+
+```mermaid
+flowchart TD
+    A[Client] -->|GET /images/| B[FastAPI Application]
+    B --> C[Images Router]
+    C --> D[get_all_images]
+    D -->|tagged filter| E[db_get_all_images]
+    E --> F[(SQLite Database)]
+    F -->|Images + Tags| E
+    E --> G1[image_util_parse_metadata]
+    G1 --> E
+    E --> D
+    D --> G2[image_util_parse_metadata]
+    G2 --> H[ImageData]
+    H --> I[GetAllImagesResponse]
+    I -->|JSON Response| A
+```
 <div class="api-500-wrapper">
 	<swagger-ui src="openapi.json"/>
 </div>