Upgrading from 26.8 to 26.9 failed (could not create unique index "perf_unique_org_string_usecase")

### Self-Hosted Version

26.9

### CPU Architecture

x86_64

### Docker Version

26.1.4

### Docker Compose Version

2.27.1

### Machine Specification

- [x] My system meets the minimum system requirements of Sentry

### Installation Type

Upgrade from 26.8 to 26.9

### Steps to Reproduce

```
▶ Ensuring proper PostgreSQL version ...
Container sentry-self-hosted-postgres-1 Creating
Container sentry-self-hosted-postgres-1 Created
Container sentry-self-hosted-postgres-1 Starting
Container sentry-self-hosted-postgres-1 Started
Container sentry-self-hosted-postgres-1 Waiting
Container sentry-self-hosted-postgres-1 Healthy
Re-indexing due to glibc change, this may take a while...
ERROR: could not create unique index "perf_unique_org_string_usecase"
DETAIL: Key (string, organization_id, use_case_id)=(d:transactions/measurements.cls@ratio, 1, transactions) is duplicated.
Error in install/upgrade-postgres.sh:53.
'$dc exec postgres psql -U postgres -v ON_ERROR_STOP=1 -c "REINDEX DATABASE postgres;"' exited with status 1
-> ./install.sh:main:41
--> install/upgrade-postgres.sh:source:53

Cleaning up...
```

### Expected Result

I should be able to upgrade from 26.8 to 26.9.

### Actual Result

I cannot upgrade from 26.8 to 26.9

### Event ID

_No response_

Start with install/upgrade-postgres.sh at line 53 and install.sh at line 41, then review the PostgreSQL upgrade and reindex commands shown in the failure. Reproduce the 26.8-to-26.9 upgrade with the duplicate key data, identify how the unique index migration should handle it, and verify that the upgrade completes successfully.

Written by the indexing model from the issue text.