Link pipeline creatives to RRSS calendar pieces with pick-or-create UI.

Agency can choose a scheduled calendar post to prefill title, copy, pillar and date, or create/edit one in the calendar while adding it to the pipeline.

Co-authored-by: Cursor <cursoragent@cursor.com>

sql/creative_pipeline_calendar_link.sql

@@ -0,0 +1,12 @@
+-- Link pipeline creatives to RRSS calendar pieces (documents content.pieces[].id).
+-- Idempotent.
+
+alter table public.creative_pipeline
+  add column if not exists calendar_piece_id text;
+
+create index if not exists creative_pipeline_calendar_piece_idx
+  on public.creative_pipeline (tenant_id, calendar_piece_id)
+  where calendar_piece_id is not null;
+
+comment on column public.creative_pipeline.calendar_piece_id is
+  'Stable id of the RRSS calendar piece (documents type=calendar → pieces[].id). Agency links pipeline ↔ editorial plan.';