perf(webapp): avoid unindexed fileId scan in get-background-worker-by-version (#4245)

## What

The `GET
/api/v1/projects/:projectRef/background-workers/:envSlug/:version`
endpoint loaded each file's tasks through the nested `files.tasks`
relation. Prisma resolves that as a separate query:

```sql
SELECT id, slug, "fileId" FROM "BackgroundWorkerTask" WHERE "fileId" IN (...)
```

`BackgroundWorkerTask.fileId` is not indexed — the FK constraint exists,
but Postgres does not auto-create an index for foreign keys — so on a
large table this can only run as a sequential scan, which gets
progressively slower as the table grows and was observed taking minutes
per call in production.

The loader already loads every task for the worker via `tasks: true`,
which uses the indexed `workerId` relation, and those rows already
include `fileId`. This PR groups task slugs by `fileId` in memory from
that already-loaded data and drops the `files.tasks` include entirely.

## Behavior change (latent bug fix)

The response shape is unchanged, but there is a semantic correction for
**source files reused across worker versions** (files are de-duplicated
by `@@unique([projectId, contentHash])`, so one file row can be linked
to many workers).

- **Before:** `file.tasks` came from the `BackgroundWorkerFile.tasks`
relation, i.e. *every* `BackgroundWorkerTask` with that `fileId` —
across all workers sharing the file. So a worker's manifest could list
tasks it doesn't actually have.
- **After:** `file.tasks` is grouped from the queried worker's own
tasks, so it reflects only that worker version's tasks.

Verified on a local DB: 460 files are referenced by tasks from more than
one worker; of 6819 (worker, file) pairs, 6 differ — all one file where
the old union leaked a task slug (`cancellation-test`) into worker
versions that never had it. The new per-worker behavior is the correct
one for a worker-version manifest. (Thanks to the automated review for
flagging this.)

## Analysis

Captured the exact SQL before/after by instrumenting Prisma against real
data (a worker with 62 files):

- **Before:** 5 statements, including the `WHERE "fileId" IN (...)`
scan.
- **After:** 4 statements; the `fileId` query is gone and the other four
are identical.

EXPLAIN of the two access paths:

```
Before  WHERE "fileId" IN (...)
  Seq Scan on "BackgroundWorkerTask"
    Filter: ("fileId" = ANY (...))          -- reads the whole table, scales with table size

After   WHERE "workerId" IN (...)
  Index Scan using "BackgroundWorkerTask_workerId_slug_key"
    Index Cond: ("workerId" = ...)          -- bounded by matching rows, scale-independent
```

No new index is required: the `workerId` access path is already covered
by the existing `BackgroundWorkerTask_workerId_slug_key` unique index.

## Testing

- `pnpm run typecheck --filter webapp` passes.
- Query capture + EXPLAIN performed against a local database seeded with
real worker/file/task data.

.server-changes/background-worker-version-endpoint-tasks.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Speed up retrieving a background worker by version. The endpoint no longer runs a slow lookup that scanned the full task table for large deployments; it now reuses data it already loads, so the response is the same but returns much faster.

apps/webapp/app/routes/api.v1.projects.$projectRef.background-workers.$envSlug.$version.ts

@@ -45,15 +45,7 @@ export async function loader({ params, request }: LoaderFunctionArgs) {
       },
       include: {
         tasks: true,
-        files: {
-          include: {
-            tasks: {
-              select: {
-                slug: true,
-              },
-            },
-          },
-        },
+        files: true,
       },
     });
 
@@ -61,6 +53,19 @@ export async function loader({ params, request }: LoaderFunctionArgs) {
       return json({ error: "Background worker not found" }, { status: 404 });
     }
 
+    // Group task slugs by fileId from the already-loaded tasks (which are fetched
+    // via the indexed workerId relation) instead of loading files.tasks, which
+    // queries BackgroundWorkerTask by the unindexed fileId column.
+    const taskSlugsByFileId = new Map<string, Set<string>>();
+    for (const task of backgroundWorker.tasks) {
+      if (!task.fileId) {
+        continue;
+      }
+      const slugs = taskSlugsByFileId.get(task.fileId) ?? new Set<string>();
+      slugs.add(task.slug);
+      taskSlugsByFileId.set(task.fileId, slugs);
+    }
+
     return json({
       id: backgroundWorker.friendlyId,
       version: backgroundWorker.version,
@@ -82,7 +87,7 @@ export async function loader({ params, request }: LoaderFunctionArgs) {
         filePath: file.filePath,
         contentHash: file.contentHash,
         contents: decompressContent(file.contents),
-        tasks: Array.from(new Set(file.tasks.map((task) => task.slug))),
+        tasks: Array.from(taskSlugsByFileId.get(file.id) ?? []),
       })),
     });
   } catch (error) {