chore: rename Stirling-Image to ashim across entire codebase

Complete rebrand from Stirling-Image to ashim following the project
move to https://github.com/ashim-hq/ashim.

Changes across 117 files:
- Package scope: @stirling-image/* → @ashim/*
- GitHub URLs: stirling-image/stirling-image → ashim-hq/ashim
- Docker Hub: stirlingimage/stirling-image → ashimhq/ashim
- GitHub Pages: stirling-image.github.io → ashim-hq.github.io
- All branding text: "Stirling Image" → "ashim"
- Docker service/volumes/user: stirling → ashim
- Database: stirling.db → ashim.db
- localStorage keys: stirling-token → ashim-token
- Environment variables: STIRLING_GPU → ASHIM_GPU
- Python cache dirs: .cache/stirling-image → .cache/ashim
- SVG filter IDs, test prefixes, and all other references

apps/web/public/manifest.json

@@ -1,6 +1,6 @@
 {
-  "name": "Stirling Image",
-  "short_name": "Stirling Image",
+  "name": "ashim",
+  "short_name": "ashim",
   "icons": [
     { "src": "/logo-192.png", "sizes": "192x192", "type": "image/png" },
     { "src": "/logo-512.png", "sizes": "512x512", "type": "image/png" }