database/setup.sh fails on fresh PostgreSQL: CREATE DATABASE inside DO $$ block

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## Summary

`database/setup.sh` fails on a fresh PostgreSQL instance when running `database/seeds/001-install.sql`.

The `marcus` role is created, but the `application` database is not. The next steps in `setup.sh` then fail because the database does not exist.

## Steps to reproduce

```bash
git clone https://github.com/metarhia/Example.git
cd Example

docker run --rm --name pgtest -e POSTGRES_PASSWORD=test -d postgres:17-alpine
until docker exec pgtest pg_isready -U postgres; do sleep 1; done

docker cp database/seeds/001-install.sql pgtest:/tmp/001-install.sql
docker exec pgtest psql -U postgres -v ON_ERROR_STOP=1 -f /tmp/001-install.sql
```

## Actual result

```
DO
psql:/tmp/001-install.sql:15: ERROR: CREATE DATABASE cannot be executed from a function
CONTEXT: SQL statement "CREATE DATABASE application OWNER marcus"
PL/pgSQL function inline_code_block line 4 at SQL statement
```

## Cause

PostgreSQL does not allow `CREATE DATABASE` inside a PL/pgSQL `DO $$` block. Reproduced on PostgreSQL 12–17.

## Context

Related to #297: seeds were added for docker-compose, where `POSTGRES_DB=application` created the database before init scripts ran, so the failing block in `001-install.sql` was masked. After #305 (docker removed), `database/setup.sh` is the main manual path, but it fails on a clean database.

## Suggested fix

Replace the second `DO $$` block in `database/seeds/001-install.sql` with:

```sql
DO $$
BEGIN
IF NOT EXISTS (SELECT 1 FROM pg_roles WHERE rolname = 'marcus') THEN
CREATE ROLE marcus LOGIN PASSWORD 'marcus';
END IF;
END
$$;

SELECT 'CREATE DATABASE application OWNER marcus'
WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'application')\gexec
```

(`\gexec` is a psql meta-command; `setup.sh` already uses `psql -f`.)

Start with database/seeds/001-install.sql, focusing on the second DO $$ block, then review database/setup.sh and reproduce the failure with the provided PostgreSQL Docker and psql commands. Update the seed flow so role creation remains valid and database creation works on a fresh PostgreSQL instance. Done means the seed file and setup.sh complete successfully without the missing application database error.

Written by the indexing model from the issue text.