CUB-111: design PostgreSQL schema for Extrudex Go backend

- Add migration 000001: initial schema with all lookup tables, core entities,
  and app settings table
- Add migration 000002: seed data for printer types, job statuses, material
  bases, finishes, modifiers, and default settings
- Add Go model structs in internal/models with json tags and pointer types
  for nullable fields
- Add go.mod with pgx dependency

Key decisions:
- FK ON DELETE: RESTRICT on material_base/finish/printer, SET NULL on
  optional parents (modifier, spool on print_jobs), CASCADE for usage_logs
- Soft-delete (deleted_at) on spools and print_jobs
- Lookup tables for printer_type and job_status (no raw enums)
- Partial indexes for active spools, barcode scans, low-stock queries
- Settings table with JSONB values for flexible app config
- All identifiers snake_case

backend/migrations/000002_seed_data.down.sql

@@ -0,0 +1,15 @@
+-- Migration: 000002_seed_data (rollback)
+-- Description: Remove seed data inserted in 000002
+-- Author: Hex
+-- Date: 2026-05-06
+
+BEGIN;
+
+DELETE FROM settings            WHERE key IN ('default_low_stock_threshold_grams', 'default_diameter_mm', 'filament_cross_section_area_mm2');
+DELETE FROM material_modifiers  WHERE id IN (1, 2, 3, 4);
+DELETE FROM material_finishes   WHERE id IN (1, 2, 3, 4, 5);
+DELETE FROM material_bases      WHERE id IN (1, 2, 3, 4, 5, 6, 7);
+DELETE FROM job_statuses        WHERE id IN (1, 2, 3, 4, 5, 6);
+DELETE FROM printer_types       WHERE id IN (1, 2);
+
+COMMIT;