docs: add SEO descriptions to all doc pages and update hero subtitle

apps/docs/api/image-engine.md

@@ -1,3 +1,7 @@
+---
+description: Image engine operations reference. All Sharp-based image processing operations and their parameters.
+---
+
 # Image engine
 
 The `@snapotter/image-engine` package handles all non-AI image operations. It wraps [Sharp](https://sharp.pixelplumbing.com/) and runs entirely in-process with no external dependencies.

apps/docs/api/rest.md

@@ -1,3 +1,7 @@
+---
+description: Complete REST API reference. Tool endpoints, batch processing, pipelines, file library, authentication, teams, and admin operations.
+---
+
 # REST API Reference
 
 Interactive API docs with request/response examples are available at [http://localhost:1349/api/docs](http://localhost:1349/api/docs).