Add automatic DB migration runner on app startup

[Copilot]

No mechanism existed to evolve the database schema over time — the init.sql approach only runs once on a fresh volume and cannot apply incremental changes to an existing database.

Approach

• Migration runner (src/lib/server/migrate.js): on each startup, creates a schema_migrations tracking table if absent, then applies any .sql files from database/migrations/ not yet recorded — in sorted filename order, each wrapped in a transaction.
• Server hook (src/hooks.server.js): triggers runMigrations() at module load; all incoming requests await completion, so a failed migration surfaces as request errors rather than silent schema drift.
• Initial migration (database/migrations/V001__initial_schema.sql): existing schema from init.sql converted to IF NOT EXISTS — idempotent against databases already initialised by the old mechanism.
• Dockerfile: copies database/ into the production runner stage so migration files are available at runtime.
• docker-compose.yml: removes the init.sql docker-entrypoint-initdb.d volume mount; migrations are now the sole owner of schema creation.

Adding future migrations

Drop a new .sql file into database/migrations/ with a name that sorts after existing ones (e.g. V002__add_column.sql). It will be applied automatically on next startup.

Backward compatibility

Existing databases (initialised via the old init.sql) are handled safely: V001 uses IF NOT EXISTS, so the first run records it as applied with no destructive effect.

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