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/20260518000001_usage_logs.sql

@@ -0,0 +1,37 @@
+-- Usage tracking tables migrated from the Node.js backend.
+
+CREATE TABLE IF NOT EXISTS usage_logs (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    downstream_key_id INTEGER,
+    model TEXT,
+    prompt_tokens INTEGER DEFAULT 0,
+    completion_tokens INTEGER DEFAULT 0,
+    total_tokens INTEGER DEFAULT 0,
+    cost REAL DEFAULT 0.0,
+    created_at TEXT DEFAULT (datetime('now')),
+    FOREIGN KEY (downstream_key_id) REFERENCES downstream_api_keys(id) ON DELETE SET NULL
+);
+
+CREATE INDEX IF NOT EXISTS usage_logs_downstream_key_created_idx
+    ON usage_logs (downstream_key_id, created_at);
+
+CREATE INDEX IF NOT EXISTS usage_logs_created_idx
+    ON usage_logs (created_at);
+
+CREATE TABLE IF NOT EXISTS usage_daily_snapshots (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    local_day TEXT NOT NULL,
+    downstream_key_id INTEGER,
+    request_count INTEGER NOT NULL DEFAULT 0,
+    prompt_tokens INTEGER NOT NULL DEFAULT 0,
+    completion_tokens INTEGER NOT NULL DEFAULT 0,
+    total_tokens INTEGER NOT NULL DEFAULT 0,
+    total_cost REAL NOT NULL DEFAULT 0.0,
+    created_at TEXT DEFAULT (datetime('now')),
+    updated_at TEXT DEFAULT (datetime('now')),
+    UNIQUE(local_day, downstream_key_id),
+    FOREIGN KEY (downstream_key_id) REFERENCES downstream_api_keys(id) ON DELETE SET NULL
+);
+
+CREATE INDEX IF NOT EXISTS usage_daily_snapshots_day_idx
+    ON usage_daily_snapshots (local_day);