Add modular app structure for L1 server refactoring

Phase 2 of the full modernization:
- App factory pattern with create_app()
- Settings via dataclass with env vars
- Dependency injection container
- Router stubs for auth, storage, api, recipes, cache, runs
- Service layer stubs for run, recipe, cache
- Repository layer placeholder

Routes are stubs that import from legacy server.py during migration.
Next: Migrate each router fully with templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routers/cache.py

@@ -0,0 +1,55 @@
+"""
+Cache and media routes for L1 server.
+
+Handles content retrieval, metadata, media preview, and publishing.
+"""
+
+from fastapi import APIRouter, Request, Depends, HTTPException
+from fastapi.responses import HTMLResponse, FileResponse
+
+from artdag_common.middleware import UserContext, wants_html
+
+from ..dependencies import require_auth, get_templates, get_current_user
+
+router = APIRouter()
+
+
+# TODO: Migrate routes from server.py lines 2767-4200
+# - GET /cache/{content_hash} - Get content details
+# - GET /cache/{content_hash}/raw - Download raw file
+# - GET /cache/{content_hash}/mp4 - Video conversion
+# - GET /cache/{content_hash}/meta - Get metadata
+# - PATCH /cache/{content_hash}/meta - Update metadata
+# - POST /cache/{content_hash}/publish - Publish to L2
+# - PATCH /cache/{content_hash}/republish - Republish
+# - DELETE /cache/{content_hash} - Delete content
+# - POST /cache/import - Import from IPFS
+# - POST /cache/upload - Upload content
+# - GET /media - Media list
+
+
+@router.get("/cache/{content_hash}")
+async def get_cache_item(
+    content_hash: str,
+    request: Request,
+):
+    """Get cached content details or serve the file."""
+    # TODO: Implement with content negotiation
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.get("/cache/{content_hash}/raw")
+async def download_raw(content_hash: str):
+    """Download the raw cached file."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.get("/media")
+async def list_media(
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """List all media in cache."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")