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

.env.example

@@ -14,9 +14,9 @@ MAX_MEGAPIXELS=100
 RATE_LIMIT_PER_MIN=100
 # Set to true in CI/dev to skip the forced password-change on the default admin
 # SKIP_MUST_CHANGE_PASSWORD=false
-DB_PATH=./data/stirling.db
+DB_PATH=./data/ashim.db
 WORKSPACE_PATH=./tmp/workspace
 FILES_STORAGE_PATH=./data/files
 DEFAULT_THEME=light
 DEFAULT_LOCALE=en
-APP_NAME=Stirling Image
+APP_NAME=ashim