fix: rename EnablePostGIS migration to valid TypeORM filename

[kilodesodiq-arch]

Summary

The EnablePostGIS migration existed in the repository but was saved under an invalid filename (…EnablePostGIS.ts, using a Unicode ellipsis character). TypeORM migration discovery relies on glob pattern matching — this malformed filename caused the migration to be silently skipped, leaving PostGIS uninstalled and all spatial queries failing.

This PR renames the file to the correct TypeORM convention: 1700000000000-EnablePostGIS.ts. The timestamp prefix ensures it runs before CreateGistsTable.ts (timestamp 1700000000001), satisfying the dependency order required by PostGIS spatial functions.

Changes

• Renamed Backend/src/database/migrations/…EnablePostGIS.ts → Backend/src/database/migrations/1700000000000-EnablePostGIS.ts
• File contents are unchanged — the migration correctly enables both postgis and postgis_topology extensions

Testing

• Migration file is now discoverable by TypeORM CLI (npm run migration:run)
• 1700000000000-EnablePostGIS.ts runs before 1700000000001-CreateGistsTable.ts, satisfying the PostGIS dependency
• Existing tests unaffected

Closes #32

[snowrugar-beep]

LGTM