[Tech Debt] Add FK constraint on comments.plot_index

[realproject7]

Problem

The comments table has storyline_id referencing storylines(storyline_id), but no compound constraint ensuring the plot_index actually exists in the plots table. Users could post comments on non-existent plot indexes.

Fix

Add a check or FK to validate plot existence. Options:

1. Application-level check in POST /api/comments: query plots table to verify the (storyline_id, plot_index) pair exists before inserting
2. DB trigger/function: validate on insert

Option 1 is simpler and sufficient.

Acceptance Criteria

• POST /api/comments returns 400 if (storyline_id, plot_index) doesn't exist in plots