feat(finance): Phase 4 — Tags

- tags table: name, color; transaction_tags junction table
- GET/POST /api/tags; DELETE /api/tags/:id
- POST/DELETE /api/transactions/:id/tags for per-transaction tagging
- Bulk tag/untag via /api/transactions/bulk (action: tag/untag)
- Tags returned inline with transaction list via LATERAL join
- Tag filter on Transactions page
- Bulk "Tag as..." in bulk action bar
- Tag pills + "+" picker on each transaction row
- /tags page: create with color picker, list with counts, delete

prisma/schema.prisma

@@ -15,3 +15,53 @@ model transaction_overrides {
   notes               String?
   updated_at          DateTime  @default(now()) @updatedAt
 }
+
+model participants {
+  id                    Int                     @id @default(autoincrement())
+  name                  String                  @unique
+  email                 String?                 @unique
+  created_at            DateTime                @default(now())
+  splits                transaction_splits[]
+  account_owner_mappings account_owner_mappings[]
+}
+
+model account_owner_mappings {
+  id             Int          @id @default(autoincrement())
+  bank_name      String
+  account_number String
+  owner_id       Int
+  created_at     DateTime     @default(now())
+  owner          participants @relation(fields: [owner_id], references: [id])
+
+  @@unique([bank_name, account_number])
+}
+
+model transaction_splits {
+  id             Int          @id @default(autoincrement())
+  transaction_id Int
+  participant_id Int
+  share_percent  Decimal      @db.Decimal(5, 2)
+  settled        Boolean      @default(false)
+  settled_at     DateTime?
+  created_at     DateTime     @default(now())
+  participant    participants @relation(fields: [participant_id], references: [id])
+
+  @@unique([transaction_id, participant_id])
+}
+
+model tags {
+  id               Int                @id @default(autoincrement())
+  name             String             @unique
+  color            String             @default("#6366f1")
+  created_at       DateTime           @default(now())
+  transaction_tags transaction_tags[]
+}
+
+model transaction_tags {
+  transaction_id Int
+  tag_id         Int
+  created_at     DateTime @default(now())
+  tag            tags     @relation(fields: [tag_id], references: [id], onDelete: Cascade)
+
+  @@id([transaction_id, tag_id])
+}