Refactor ateapi PostgreSQL pool ownership and OpenFGA dependency injection in main

[SHRUTI6991]

Context

Follow-up to #1670 based on review feedback.

Currently, atepg.Connect dials and owns the primary PostgreSQL connection pool internally, and ateapi opens a separate dedicated *pgxpool.Pool (NewPool) for authz.NewServer because OpenFGA's postgres.Datastore.Close() closes the underlying pool. Additionally, authz.NewServer constructs stateful dependencies (datastore and fgaServer) inline rather than receiving them via dependency injection.

Goals

1. Share a single primary PostgreSQL connection pool between atepg and OpenFGA:

   • Dial the *pgxpool.Pool in cmd/ateapi/main.go and manage its lifecycle at the top level (defer pool.Close()).
   • Inject the shared pool into both atepg and OpenFGA to avoid extra connection pool pressure on PostgreSQL.
   • Ensure shutdown ordering (defer LIFO in main.go) cleanly stops fgaServer and atepg background loops before closing the shared pool.

2. Manage OpenFGA datastore and fgaServer lifecycles in main.go:

   • Expose initialization helpers in internal/authz (e.g., ApplyMigrations, store/model bootstrapping) rather than constructing and closing datastore and fgaServer inline inside authz.NewServer.
   • Use unconditional defer statements in cmd/ateapi/main.go for datastore.Close() and fgaServer.Close() so panics or startup errors cannot leak open resources.

3. Simplify internal/authz wrapper struct:

   • Re-evaluate the authz.Server wrapper alongside control-plane API authorization call sites (Check, Write, Read) so it only encapsulates what callers need (storeID, modelID, and domain helpers).

Acceptance Criteria

• cmd/ateapi/main.go dials the shared *pgxpool.Pool and injects it into atepg and OpenFGA.
• atepg.Persistence.NewPool is removed in favor of the shared injected pool.
• datastore and fgaServer lifecycles are managed via defer in main.go.
• Unit and E2E tests pass with the shared pool configuration.