feat: production release with React SPA, file attachments, and deploy tooling

Backend:
- Add file_handlers.go: presigned upload/download for item attachments
- Add item_files.go: item file and thumbnail DB operations
- Add migration 011: item_files table and thumbnail_key column
- Update items/projects/relationships DB with extended field support
- Update routes: React SPA serving from web/dist, file upload endpoints
- Update auth handlers and middleware for cookie + bearer token auth
- Remove Go HTML templates (replaced by React SPA)
- Update storage client for presigned URL generation

Frontend:
- Add TagInput component for tag/keyword entry
- Add SVG assets for Silo branding and UI icons
- Update API client and types for file uploads, auth, extended fields
- Update AuthContext for session-based auth flow
- Update LoginPage, ProjectsPage, SchemasPage, SettingsPage
- Fix tsconfig.node.json

Deployment:
- Update config.prod.yaml: single-binary SPA layout at /opt/silo
- Update silod.service: ReadOnlyPaths for /opt/silo
- Add scripts/deploy.sh: build, package, ship, migrate, start
- Update docker-compose.yaml and Dockerfile
- Add frontend-spec.md design document

migrations/011_item_files.sql

@@ -0,0 +1,18 @@
+-- Migration 011: Item File Attachments
+--
+-- Adds an item_files table for multiple file attachments per item (not tied to revisions),
+-- and a thumbnail_key column on items for item-level thumbnails.
+
+CREATE TABLE item_files (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  item_id UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+  filename TEXT NOT NULL,
+  content_type TEXT NOT NULL DEFAULT 'application/octet-stream',
+  size BIGINT NOT NULL DEFAULT 0,
+  object_key TEXT NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_item_files_item ON item_files(item_id);
+
+ALTER TABLE items ADD COLUMN IF NOT EXISTS thumbnail_key TEXT;