feat: migrate to PostgreSQL + SQLAlchemy ORM, rebrand to Nexus Autoparts

- Migrate from SQLite to PostgreSQL with normalized schema
- Add 11 lookup tables (fuel_type, body_type, drivetrain, transmission,
  materials, position_part, manufacture_type, quality_tier, countries,
  reference_type, shapes)
- Rewrite dashboard/server.py (76 routes) using SQLAlchemy text() queries
- Rewrite console/db.py (27 methods) using SQLAlchemy ORM
- Add models.py with 27 SQLAlchemy model definitions
- Add config.py for centralized DB_URL configuration
- Add migrate_to_postgres.py migration script
- Add docs/METABASE_GUIDE.md with complete data entry guide
- Rebrand from "AUTOPARTS DB" to "NEXUS AUTOPARTS"
- Fill vehicle data gaps via NHTSA API + heuristics:
  engines (cylinders, power, torque), brands (country, founded_year),
  models (body_type, production years), MYE (drivetrain, transmission, trim)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashboard/nav.js

@@ -1,5 +1,5 @@
 /**
- * nav.js  --  Shared navigation component for AutoParts DB
+ * nav.js  --  Shared navigation component for NEXUS AUTOPARTS
  *
  * Injects a consistent header/nav bar into <div id="shared-nav"></div>.
  * Auto-highlights the current page link based on window.location.pathname.
@@ -86,7 +86,7 @@
         +           '-webkit-background-clip: text;'
         +           '-webkit-text-fill-color: transparent;'
         +           'background-clip: text;'
-        +       '">AUTOPARTS DB</span>'
+        +       '">NEXUS AUTOPARTS</span>'
         +     '</a>'
         // Slot for extra page-specific content (search bars, stats, etc.)
         +     '<div id="shared-nav-extra" style="display: contents;"></div>'