feat(inventory): multi-colour gradients, transparency, visual effects (#1154)

Spool and color_catalog rows carry extra_colors (comma-separated hex
  stops) and effect_type (14 visual variants: surface effects, sheen,
  structural). The shared FilamentSwatch component renders gradient,
  conic, effect overlay, and alpha-checkerboard consistently across the
  inventory grid, table, group banner, card, ColorSection preview, and
  catalog editor. Catalog hex_color accepts #RRGGBBAA so catalog entries
  can carry transparency too.

  The paste field accepts the exact format 3dfilamentprofiles.com puts on
  its filament details pages, so users can copy a multi-colour combo
  directly. The effect dropdown spans the full filament-variant
  vocabulary -- surface effects (sparkle/wood/marble/glow/matte), sheen
  variants (silk/galaxy/rainbow/metal/translucent), and structural
  variants (gradient/dual-color/tri-color/multicolor). None of these
  fields touch MQTT/firmware -- pure visual hint.

  Spool group-key extended to include extra_colors + effect_type so
  "Group similar" no longer collapses visually distinct spools.

  Migrations: 4 idempotent ALTER TABLE ADD COLUMN (Postgres-safe), plus
  ALTER COLUMN hex_color TYPE VARCHAR(9) on Postgres only (SQLite ignores
  VARCHAR length).

  Tests: 42 new backend (35 unit + 7 integration), 20 new frontend (14
  FilamentSwatch + 3 ColorCatalogSettings + 3 InventoryPageGrouping
  regression). 3522 backend + 1582 frontend tests pass; ruff clean.
  Localised across all 8 UI locales.

frontend/src/components/SpoolFormModal.tsx

@@ -269,6 +269,8 @@ export function SpoolFormModal({
           brand: spool.brand || '',
           color_name: spool.color_name || '',
           rgba: validRgba,
+          extra_colors: spool.extra_colors || '',
+          effect_type: spool.effect_type || '',
           label_weight: spool.label_weight || 1000,
           core_weight: spool.core_weight || 250,
           core_weight_catalog_id: spool.core_weight_catalog_id ?? null,
@@ -518,6 +520,8 @@ export function SpoolFormModal({
       brand: formData.brand || null,
       color_name: formData.color_name || null,
       rgba: formData.rgba || null,
+      extra_colors: formData.extra_colors || null,
+      effect_type: formData.effect_type || null,
       label_weight: formData.label_weight,
       core_weight: formData.core_weight,
       core_weight_catalog_id: formData.core_weight_catalog_id,