Include platform in content_block_pk surrogate (dim_course_content and children) #2384

Description/Context

dim_course_content generates content_block_pk from ['block_id', 'retrieved_at']. README.md:116 claims block_id encodes the run, but README.md:36-45 itself documents that migrated courses keep identical readable IDs (hence identical block_ids) across two platforms. The unique test only holds while the platforms' retrieved_at values differ.

dim_video and dim_problem inherit the non-unique PKs (their yml carries "NOT globally unique" warnings — a dimension PK that needs a second column to be unique is not a surrogate).

Plan/Design

Include platform in the surrogate for dim_course_content, dim_video, dim_problem, and the dim_discussion_topic-adjacent content dims. Coordinate downstream FK regeneration.

P2. Part of #2375 (2026-07 dbt warehouse audit). Witan task: tk-include-platform-in-content-block-pk-surrogate-d-984a44.