refactor: purge Node.js, Rust is the one true backend

- Moved boosapi-rs/* to repo root
- Deleted all Node.js source (src/server/, src/web/, scripts/)
- Deleted node_modules, package.json, tsconfig, vite.config
- Updated .gitignore and AGENTS.md for Rust-only stack
- Fixed static file path for frontend dist/web/
- Dockerfile stays the same (already Rust-only)
- Kept dist/web/ for Rust static serving

migrations/20260517000001_background_tasks.sql

@@ -0,0 +1,24 @@
+-- Background tasks tracking
+CREATE TABLE IF NOT EXISTS background_tasks (
+    id              TEXT PRIMARY KEY NOT NULL,
+    task_type       TEXT NOT NULL,
+    title           TEXT NOT NULL,
+    status          TEXT NOT NULL DEFAULT 'pending',
+    message         TEXT DEFAULT '',
+    error           TEXT,
+    dedupe_key      TEXT,
+    success_title   TEXT,
+    failure_title   TEXT,
+    notify_on_success INTEGER DEFAULT true,
+    notify_on_failure INTEGER DEFAULT true,
+    result_json     TEXT,
+    progress        INTEGER DEFAULT 0,
+    progress_total  INTEGER DEFAULT 0,
+    created_at      TEXT DEFAULT (datetime('now')),
+    updated_at      TEXT DEFAULT (datetime('now')),
+    started_at      TEXT,
+    finished_at     TEXT
+);
+
+CREATE INDEX IF NOT EXISTS background_tasks_status_idx
+    ON background_tasks (status);