feat(plan): add plan_categories table, migrate category enum to varchar

- Add `plan_categories` model (id, key, label, color, sort_order, is_active)
- Change `work_plan_entries.category` from ENUM plan_category to VARCHAR(50)
- Change `work_plan_overrides.category` from ENUM plan_category to VARCHAR(50)
- Remove the `plan_category` enum from schema
- Seed 7 built-in categories (work, preparation, travel, leave, sick, training, other)

Also fixes two pre-existing migration issues that blocked shadow-DB execution:
- 20260605122718: swap CREATE INDEX before DROP INDEX so MySQL FK constraint is not broken
- 20260514071455_init: fix quotations.project_code VARCHAR(50→100) to match actual DB schema

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

prisma/migrations/20260606002901_plan_categories/migration.sql

@@ -0,0 +1,32 @@
+-- Add plan_categories table and convert category columns from ENUM to VARCHAR(50)
+
+-- CreateTable
+CREATE TABLE `plan_categories` (
+    `id` INTEGER NOT NULL AUTO_INCREMENT,
+    `key` VARCHAR(50) NOT NULL,
+    `label` VARCHAR(100) NOT NULL,
+    `color` VARCHAR(7) NOT NULL,
+    `sort_order` INTEGER NOT NULL DEFAULT 0,
+    `is_active` BOOLEAN NOT NULL DEFAULT true,
+    `created_at` DATETIME(0) NULL DEFAULT CURRENT_TIMESTAMP(0),
+    `updated_at` DATETIME(0) NULL DEFAULT CURRENT_TIMESTAMP(0),
+
+    UNIQUE INDEX `plan_categories_key_key`(`key`),
+    PRIMARY KEY (`id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+-- AlterTable: change category from ENUM to VARCHAR(50) in work_plan_entries
+ALTER TABLE `work_plan_entries` MODIFY `category` VARCHAR(50) NOT NULL DEFAULT 'work';
+
+-- AlterTable: change category from ENUM to VARCHAR(50) in work_plan_overrides
+ALTER TABLE `work_plan_overrides` MODIFY `category` VARCHAR(50) NOT NULL;
+
+-- Seed the categories that were previously the plan_category enum
+INSERT INTO `plan_categories` (`key`, `label`, `color`, `sort_order`, `is_active`) VALUES
+  ('work',        'Práce',          '#2563eb', 1, true),
+  ('preparation', 'Příprava',       '#0d9488', 2, true),
+  ('travel',      'Cesta / Montáž', '#ca8a04', 3, true),
+  ('leave',       'Dovolená',       '#16a34a', 4, true),
+  ('sick',        'Nemoc',          '#dc2626', 5, true),
+  ('training',    'Školení',        '#7c3aed', 6, true),
+  ('other',       'Jiné',           '#6b7280', 7, true);