fix(flowsheet): switch nextPlayOrder index to composite (show_id, play_order DESC)

[jakebromberg]

Summary

flowsheet_play_order_idx (migration 0073) was introduced as a single-column DESC index on play_order to back the then-global SELECT max(play_order) FROM flowsheet that nextPlayOrder() ran on every flowsheet insert — correctly O(1) at the top of the index. #693 then rewrote nextPlayOrder() to scope by show_id, leaving the index misaligned with the actual query shape: on the 2.6M-row flowsheet table the planner now has three choices, none of them O(1):

1. Backward-scan the global DESC index and filter by show_id — fast for the current show, increasingly slow as the show recedes from the leading edge.
2. Bitmap-scan flowsheet_show_id_idx (0068) + Bitmap Heap Scan + Aggregate — multiple heap reads per show, well beyond 0073's measured 0.119 ms.
3. Seq scan + filter for small shows the planner mis-estimates.

This PR swaps in a composite (show_id, play_order DESC) index that makes the per-show MAX a true O(1) leaf-page lookup: each show's rows form a contiguous run inside the index, with DESC ordering placing the per-show max at the run's leading edge. The same shape also covers two related per-show queries that the misaligned single-column index never served well either:

• apps/backend/middleware/legacy/flowsheet.mirror.ts's show_start/show_end announcement lookups (WHERE show_id = ? ORDER BY play_order DESC LIMIT 1).
• apps/backend/services/flowsheet.service.ts:getEntriesByShow (WHERE show_id IN (...) ORDER BY play_order DESC).

The predecessor flowsheet_play_order_idx is dropped in the same migration. The only remaining global-MAX(play_order) caller is jobs/flowsheet-etl/job.ts:resetSequences() — a one-shot post-bulk-load sequence reset that is not on a hot request path, so the planner falling back to a seq scan there is acceptable and the cleanup reclaims the 17 MB the single-column index used.

Changes

• New migration 0094_flowsheet-show-id-play-order-idx.sql: DROP INDEX IF EXISTS flowsheet_play_order_idx + CREATE INDEX IF NOT EXISTS flowsheet_show_id_play_order_idx ON flowsheet USING btree (show_id, play_order DESC). Same IF [NOT] EXISTS + CONCURRENTLY-out-of-band runbook pattern as 0068 / 0070 / 0074 / 0078 / 0080.
• shared/database/src/schema.ts: replace the flowsheet_play_order_idx declaration with flowsheet_show_id_play_order_idx, with a comment block explaining the misalignment history and the two additional query shapes the composite covers.
• tests/unit/database/schema.flowsheet-show-id-play-order-idx.test.ts: new regression test pinning the migration shape (DROP + CREATE composite, no CONCURRENTLY in DDL), the schema declaration (column order + DESC ordering), and the nextPlayOrder() query shape (WHERE show_id = ?). Drift on either side trips the test before reaching prod.

Production ops

Run out-of-band BEFORE merging this PR so the in-migration DDL is a no-op:

CREATE INDEX CONCURRENTLY "flowsheet_show_id_play_order_idx"
  ON "wxyc_schema"."flowsheet"
  USING btree ("show_id", "play_order" DESC);

-- After the new index is VALID:
DROP INDEX CONCURRENTLY IF EXISTS "wxyc_schema"."flowsheet_play_order_idx";

CREATE INDEX CONCURRENTLY takes only a ShareUpdateExclusiveLock, so DJs continue inserting flowsheet rows during the build. Expected size ~70 MB; build typically completes in under a minute on the prod RDS instance.

Test plan

• npm run lint (0 errors)
• npm run format:check
• npm run typecheck
• npm run test:unit (3158/3158 passing, including the new schema test + the pre-existing nextPlayOrder regression test from nextPlayOrder() should scope to current show_id, not global max #693)
• Post-merge: verify the auto-deploy succeeded; CLAUDE.md cadence rule for shared/database/src/migrations/** changes

Closes #1133

[github-actions]

Schema constraint shape report

data-shape report errored (exit 0): node:internal/modules/runmain:107 triggerUncaughtException( ^ Error ERRMODULENOTFOUND: Cannot find package 'postgres' imported from /home/runner/work/Backend-Service/Backend-Service/scripts/schema-shape-report.mjs Did you mean to import "postgres/cjs/src/index.js"? at Object.getPackageJ; manual check required