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/schema.prisma

@@ -678,16 +678,6 @@ enum sklad_inventory_status {
   CONFIRMED
 }
 
-enum plan_category {
-  work
-  preparation
-  travel
-  leave
-  sick
-  training
-  other
-}
-
 model sklad_categories {
   id          Int       @id @default(autoincrement())
   name        String    @db.VarChar(100)
@@ -934,13 +924,24 @@ model sklad_inventory_lines {
   @@map("sklad_inventory_lines")
 }
 
+model plan_categories {
+  id         Int       @id @default(autoincrement())
+  key        String    @unique @db.VarChar(50)
+  label      String    @db.VarChar(100)
+  color      String    @db.VarChar(7)
+  sort_order Int       @default(0)
+  is_active  Boolean   @default(true)
+  created_at DateTime? @default(now()) @db.DateTime(0)
+  updated_at DateTime? @default(now()) @updatedAt @db.DateTime(0)
+}
+
 model work_plan_entries {
   id         Int             @id @default(autoincrement())
   user_id    Int
   date_from  DateTime        @db.Date
   date_to    DateTime        @db.Date
   project_id Int?
-  category   plan_category   @default(work)
+  category   String          @default("work") @db.VarChar(50)
   note       String          @db.VarChar(500)
   created_by Int
   created_at DateTime?       @default(now()) @db.Timestamp(0)
@@ -961,7 +962,7 @@ model work_plan_overrides {
   user_id    Int
   shift_date DateTime      @db.Date
   project_id Int?
-  category   plan_category
+  category   String        @db.VarChar(50)
   note       String        @db.VarChar(500)
   created_by Int
   created_at DateTime?     @default(now()) @db.Timestamp(0)