feat: Background tasks table + tasks route migrated to Rust

- New migration: background_tasks table
- New model: BackgroundTask (background_task.rs)
- GET /api/tasks with limit param
- GET /api/tasks/{id} with 404 handling
- 18/18 tests pass

boosapi-rs/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);