Fix stale image-only copy and add 1.x-to-2.0 migration guide (#454)

* docs: add 1.x-to-2.0 migration guide and upgrade notice

Adds MIGRATING.md with backup and upgrade steps, plus a short
"coming from 1.x?" callout in README and the docs upgrade guide
pointing existing users at it.

* fix: replace stale image-only and pre-rename data copy across product

SnapOtter grew from an image-only tool into a 5-modality suite
(Image, Video, Audio, PDF, Files), but copy in several places never
caught up. Fixes:

- dropzone.defaultFormats (i18n): every non-English locale still had
  the pure pre-2.0 image-only format list; English omitted Files
  entirely. Corrected across all 21 locales.
- settings.about.appDescription (i18n): "document, and data" workflow
  copy updated to "PDF, and file" across all 21 locales.
- constants.ts: Files category's raw name was still "Data Files".
- Landing hero subtitle, JSON-LD schema, llms.txt, and 7 spots in the
  competitor-comparison pages.
- Docs: VitePress config, supported-formats, deployment, and an
  architecture.md modality-naming nit.
- OpenAPI description, root package.json description/keyword, and a
  GitHub issue template dropdown option.

DOCKERHUB.md's separate "v1.x, image tools only" pre-release notice
is left untouched since 2.0 hasn't published to Docker Hub yet.

* test: update dropzone format-hint assertion to match corrected copy

The expected string still had the stale image-only/duplicated
PDF-Documents text from before the dropzone.defaultFormats fix.

apps/api/src/openapi.yaml

@@ -3,7 +3,7 @@ info:
   title: SnapOtter API
   version: 2.0.0
   description: |
-    REST API for SnapOtter, a self-hosted file processing suite with 200+ tools across image, video, audio, document, and data.
+    REST API for SnapOtter, a self-hosted file processing suite with 200+ tools across image, video, audio, PDF, and files.
 
     ## Authentication
 
@@ -24,7 +24,7 @@ servers:
 
 tags:
   - name: Tools
-    description: File processing tools spanning image, video, audio, document, and data. Each accepts a multipart file upload and returns a download URL.
+    description: File processing tools spanning image, video, audio, PDF, and files. Each accepts a multipart file upload and returns a download URL.
   - name: Batch
     description: Process multiple files through a tool in one request.
   - name: Pipelines