deps: migrate lib/pq → pgx/v5 + go mod tidy (RUK-135)

[ruko1202]

TD-12: lib/pq has been in maintenance mode since 2020. Migrated the
runtime PG driver to github.com/jackc/pgx/v5/stdlib, which is the
active fork the ecosystem has rallied behind.

What changed:

• pkg/goquestorage imports pgx/v5/stdlib (anonymous), so the "pgx"
  driver registers automatically for anyone using this package.
• New PgxDriver = "pgx" and PgxV5Driver = "pgx/v5" constants for the
  two names pgx registers itself under.
• PgDriver = "postgres" stays as a // Deprecated: alias. goquestorage
  no longer registers "postgres" — callers using this name must
  _ "github.com/lib/pq" in their own code. This is intentional:
  silently aliasing "postgres" to pgx inside the package would race
  with downstream binaries that also import lib/pq (panic on
  double-Register or silent type-mismatch in error handling,
  depending on init() order).
• goque.NewStorage's switch accepts all three names.
• internal/storages/pg/task/add_task.go handleError uses
  *pgconn.PgError + pgerrcode constants (UniqueViolation,
  InvalidTextRepresentation) instead of *pq.Error + Code.Name().
• examples/service migrated to "pgx" — both config.yaml and the
  config.go default.
• scripts/dbmodels/generate.go (dev-only jet code generator): jet's
  GenerateDSN is hard-coded to sql.Open("postgres", ...); rather
  than bring lib/pq back, we open via sqlx with "pgx" and pass jet
  the embedded *sql.DB through GenerateDB. MySQL and SQLite paths
  unchanged — jet self-imports their drivers.

TD-13 (go mod tidy): cron/v3 moved from indirect to direct (we use
it in periodic processors). pgerrcode added as a small direct dep.
lib/pq stays as indirect because go-jet/v2/postgres runtime package
pulls it transitively — out of our control without replacing jet.

Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com