feat(dimensional): replace course_order placeholder in bridge_program_course

[blarghmatey]

Context

PR #1868 added bridge_program_course mapping programs to their constituent courses. The course_order column is hardcoded to 1 for all rows because source systems (MITx Online, xPro, MicroMasters) do not expose a canonical ordering of courses within a program.

A SQL comment documents this:

course_order is set to 1 as a placeholder. The source systems do not expose a canonical ordering of courses within a program, so a meaningful sort order cannot be derived without additional business logic or manual curation.

What

Investigate whether any source system or intermediate model exposes course ordering within a program:

• int__mitxonline__coursesinprogram — check for any position/order field
• int__mitxpro__coursesinprogram — same
• int__micromasters__* — check for course sequence data
• If ordering is available, populate course_order from source
• If not, consider adding a manual curation seed table (CSV) mapping program + course to order

Acceptance Criteria

• Source system ordering capability assessed and documented
• course_order either populated from source data or from a curated seed
• bridge_program_course updated and tested