TaskRun optimizations: dropping FKs and some indexes (#3309)

## Summary

- Drop all 8 foreign key constraints on TaskRun. The run listing path is
now fully ClickHouse-backed so we no longer need Postgres to enforce
referential integrity on this table. The FK constraints add write
overhead on every insert/update with no remaining benefit. Prisma
queries are unaffected.
- Remove PostgresRunsRepository and its associated feature flag
(runsListRepository), which was the last remaining code path querying
TaskRun directly for list/count operations.
- Drop three indexes that were only useful for the Postgres run list
path and have no remaining query consumers:
- TaskRun_runtimeEnvironmentId_id_idx — was the cursor pagination index
for PostgresRunsRepository; superseded by the (runtimeEnvironmentId,
createdAt DESC) composite index
- TaskRun_scheduleId_idx — redundant with the (scheduleId, createdAt
DESC) composite index; no direct Postgres queries filter by scheduleId
alone
- TaskRun_rootTaskRunId_idx — no queries filter TaskRun by rootTaskRunId
as a WHERE clause anywhere in the codebase

All index drops use CONCURRENTLY IF EXISTS to avoid table locks in
production.

## Test plan

  - pnpm run db:migrate:deploy applies all migrations cleanly
  - pnpm run typecheck --filter webapp passes
  - Run list pages load correctly in the dashboard (ClickHouse path)
  - Scheduled task runs still trigger and appear correctly

Author: matt-aitken

apps/webapp/app/routes/admin.feature-flags.tsx

@@ -55,7 +55,6 @@ export const loader = async ({ request }: LoaderFunctionArgs) => {
 
   // Resolve env-based defaults for locked flags
   const resolvedDefaults: Record<string, string> = {
-    [FEATURE_FLAG.runsListRepository]: "clickhouse",
     [FEATURE_FLAG.taskEventRepository]: env.EVENT_REPOSITORY_DEFAULT_STORE,
   };