feat(v2/postgres): Add instrumented PostgreSQL client with query tracing

Context

Part of the LabKit v2 package decomposition from the UAM PoC (!314).

Tracking issue: gitlab-org/quality/quality-engineering/team-tasks#4285 (closed) Parent epic: gitlab-org/quality#360

What's in this MR?

• v2/postgres package: instrumented PostgreSQL client implementing app.Component
  • Exposes *sql.DB via pgx/v5/stdlib instead of *pgxpool.Pool, enabling compatibility with standard Go database tooling (ORMs, query builders, migration frameworks) and the Container Registry's load balancing code
  • Automatic OpenTelemetry query tracing via pgx's QueryTracer interface
  • Start(ctx) verifies database connectivity
  • Compile-time app.Component interface assertion

Dependencies

Depends on !337 (merged) (v2/app package).

Related

• gitlab-org/quality/quality-engineering/team-tasks#4290 (closed) (addressed: switch to database/sql interface)
• gitlab-org/quality/quality-engineering/team-tasks#4291 (closed) (follow-up: PgBouncer compatibility)
• gitlab-org/quality/quality-engineering/team-tasks#4301 (follow-up: database load balancing, now unblocked by database/sql interface)