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.up.sql

@@ -0,0 +1,95 @@
+-- Seed Data: Extrudex common reference data
+-- Author: Hex
+-- Date: 2026-05-06
+--
+-- IMPORTANT: IDs are explicitly assigned to satisfy the DEFAULT constraints:
+--   - filament_spools.material_finish_id  DEFAULT 1  ("Basic")
+--   - print_jobs.job_status_id            DEFAULT 1  ("pending")
+--
+-- Density values sourced from common manufacturer specifications.
+-- Temperature ranges are conservative/typical; users can override per-spool.
+
+BEGIN;
+
+-- ============================================================================
+-- Printer Types
+-- ============================================================================
+
+INSERT INTO printer_types (id, name) VALUES
+    (1, 'fdm'),
+    (2, 'resin')
+ON CONFLICT (id) DO NOTHING;
+
+-- Reset the sequence so future inserts start after our explicit IDs
+SELECT setval('printer_types_id_seq', GREATEST(2, (SELECT MAX(id) FROM printer_types)));
+
+-- ============================================================================
+-- Job Statuses
+-- ============================================================================
+
+INSERT INTO job_statuses (id, name) VALUES
+    (1, 'pending'),
+    (2, 'printing'),
+    (3, 'paused'),
+    (4, 'completed'),
+    (5, 'failed'),
+    (6, 'cancelled')
+ON CONFLICT (id) DO NOTHING;
+
+SELECT setval('job_statuses_id_seq', GREATEST(6, (SELECT MAX(id) FROM job_statuses)));
+
+-- ============================================================================
+-- Material Bases (common filament types)
+-- ============================================================================
+
+INSERT INTO material_bases (id, name, density_g_cm3, extrusion_temp_min, extrusion_temp_max, bed_temp_min, bed_temp_max) VALUES
+    (1, 'PLA',   1.24, 190, 220,  0,  60),
+    (2, 'PETG',  1.27, 230, 250, 70,  90),
+    (3, 'ABS',   1.04, 230, 260, 90, 110),
+    (4, 'TPU',   1.21, 220, 250,  0,  60),
+    (5, 'ASA',   1.07, 240, 260, 90, 110),
+    (6, 'Nylon', 1.14, 240, 280, 70, 100),
+    (7, 'PC',    1.20, 260, 310, 90, 120)
+ON CONFLICT (id) DO NOTHING;
+
+SELECT setval('material_bases_id_seq', GREATEST(7, (SELECT MAX(id) FROM material_bases)));
+
+-- ============================================================================
+-- Material Finishes
+-- ============================================================================
+-- ID 1 = "Basic" is the default for new spools (DEFAULT 1 constraint)
+
+INSERT INTO material_finishes (id, name, description) VALUES
+    (1, 'Basic',  'Standard solid-color filament with no special finish'),
+    (2, 'Silk',   'Glossy silk-like sheen, often used for decorative prints'),
+    (3, 'Matte',  'Flat non-reflective surface finish'),
+    (4, 'Glossy', 'High-shine reflective surface'),
+    (5, 'Satin',  'Semi-gloss between matte and glossy')
+ON CONFLICT (id) DO NOTHING;
+
+SELECT setval('material_finishes_id_seq', GREATEST(5, (SELECT MAX(id) FROM material_finishes)));
+
+-- ============================================================================
+-- Material Modifiers
+-- ============================================================================
+
+INSERT INTO material_modifiers (id, name, description) VALUES
+    (1, 'Wood-Filled',   'Contains wood fibers for natural wood-like appearance and texture'),
+    (2, 'Carbon Fiber',  'Reinforced with carbon fibers for increased stiffness and strength'),
+    (3, 'Glow-in-Dark',  'Phosphorescent additive that glows after exposure to light'),
+    (4, 'Marble',        'Contains specks for a stone-like marble appearance')
+ON CONFLICT (id) DO NOTHING;
+
+SELECT setval('material_modifiers_id_seq', GREATEST(4, (SELECT MAX(id) FROM material_modifiers)));
+
+-- ============================================================================
+-- Default Application Settings
+-- ============================================================================
+
+INSERT INTO settings (key, value, description) VALUES
+    ('default_low_stock_threshold_grams', '50', 'Default grams threshold for low-stock alerts on new spools'),
+    ('default_diameter_mm', '1.75', 'Default filament diameter for new spools (1.75mm is the modern standard)'),
+    ('filament_cross_section_area_mm2', '2.405', 'Cross-sectional area for 1.75mm filament: π × (1.75/2)²')
+ON CONFLICT (key) DO NOTHING;
+
+COMMIT;