[Task 01] DB module + migrations + meta

[lubobill1990]

Task 01 — DB module + migrations + meta

Depends on: —
Unblocks: 02, 06

Goal

Set up bun:sqlite with a versioned migration system so every later step can
just call getDb() and run prepared statements.

Scope

• New file src/lib/db.ts.
• Schema for accounts, usage_events, usage_daily,
  model_pricing, model_pricing_versions, pricing_sync_log, meta.
• Migration runner driven by meta.schema_version (start at 1).
• WAL mode, synchronous = NORMAL.
• DB path resolved from --db-path flag (default
  ~/.local/share/copilot-api/usage.sqlite); add the flag to start.ts
  argument parser.
• ensurePaths() (in src/lib/paths.ts) gains the DB directory.

Non-goals

• Any actual writes from feature code (later tasks own that).
• Pricing data seeding.

API surface

export function initDb(path: string): Database
export function getDb(): Database
export function withTransaction<T>(fn: (db: Database) => T): T

Migrations live in src/lib/migrations/001_initial.sql and are applied
in order based on filename.

Definition of Done

• bun run dev starts and creates a fresh DB with all tables.
• Re-running bun run dev does NOT re-apply migrations.
• Unit test: open in-memory DB, run migrations, verify
  meta.schema_version = 1 and that all tables exist.
• --db-path works with a custom path.

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• Source: docs/tasks/01-db-module.md
• Design: docs/design/
• Phase: foundation
• Depends on tasks: -