feat(02-02): JournalEntry models + JournalEntryService

- Add JournalEntryStatus and JournalEntrySource enums to Prisma schema
- Add JournalEntry model with maker-checker fields, self-referential reversal relation, and audit timestamps
- Add JournalEntryLine model with debit/credit Decimal(15,2) fields
- Update Account model with journalEntryLines back-relation
- Update User model with createdJournalEntries and approvedJournalEntries back-relations
- Run migration: 20260304150817_add_journal_entry_models
- Add journalEntry and journalEntryLine to TENANT_SCOPED_MODELS in prisma-tenant.ts
- Create JournalEntryService with createEntry, approveEntry, reverseEntry, getAccountBalance, getTrialBalance
- Enforce debit=credit balance using integer cents comparison (avoids float issues)
- SYSTEM source entries auto-posted; MANUAL entries start as DRAFT for maker-checker

prisma/schema.prisma

@@ -68,6 +68,19 @@ enum BillingType {
   POSTPAID
 }
 
+enum JournalEntryStatus {
+  DRAFT
+  PENDING_APPROVAL
+  APPROVED
+  POSTED
+  REVERSED
+}
+
+enum JournalEntrySource {
+  SYSTEM
+  MANUAL
+}
+
 // =============================================================================
 // MODELS
 // =============================================================================
@@ -119,6 +132,8 @@ model Account {
   createdAt     DateTime      @default(now())
   updatedAt     DateTime      @updatedAt
 
+  journalEntryLines JournalEntryLine[]
+
   /// Account codes must be unique within a tenant
   @@unique([tenantId, code])
   /// RLS-ready index — always present on tenant-scoped models
@@ -252,6 +267,11 @@ model User {
   /// Accounting periods this user has closed (as administrator)
   closedPeriods AccountingPeriod[]
 
+  /// Journal entries this user created (maker)
+  createdJournalEntries JournalEntry[] @relation("JournalEntryCreatedBy")
+  /// Journal entries this user approved (checker)
+  approvedJournalEntries JournalEntry[] @relation("JournalEntryApprovedBy")
+
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt
 
@@ -260,3 +280,68 @@ model User {
   /// RLS-ready index — always present on tenant-scoped models
   @@index([tenantId])
 }
+
+/// A JournalEntry records a double-entry accounting transaction.
+/// All financial events in the system flow through this model.
+/// Entries are IMMUTABLE — no updates or deletes. Corrections use reversing entries.
+model JournalEntry {
+  id            String             @id @default(uuid())
+  tenantId      String
+  /// Auto-generated sequential identifier per tenant per year (e.g., "JE-2026-0001")
+  entryNumber   String
+  /// The accounting date (when the economic event occurred, not necessarily createdAt)
+  date          DateTime
+  description   String
+  source        JournalEntrySource
+  status        JournalEntryStatus @default(DRAFT)
+  /// Optional reference to source record (e.g., "Invoice", "Payment")
+  referenceType String?
+  /// ID of the source record (e.g., the invoice UUID)
+  referenceId   String?
+  /// If this entry reverses another entry, this points to the original
+  reversesEntryId    String?        @unique
+  reversesEntry      JournalEntry?  @relation("JournalEntryReversal", fields: [reversesEntryId], references: [id])
+  /// If this entry has been reversed, this points to the reversing entry (back-reference)
+  reversedByEntry    JournalEntry?  @relation("JournalEntryReversal")
+  /// The user who created the entry (maker)
+  createdById   String
+  createdBy     User               @relation("JournalEntryCreatedBy", fields: [createdById], references: [id])
+  /// The user who approved the entry (checker — for MANUAL entries)
+  approvedById  String?
+  approvedBy    User?              @relation("JournalEntryApprovedBy", fields: [approvedById], references: [id])
+  approvedAt    DateTime?
+  createdAt     DateTime           @default(now())
+  updatedAt     DateTime           @updatedAt
+
+  lines         JournalEntryLine[]
+
+  /// Entry numbers must be unique within a tenant
+  @@unique([tenantId, entryNumber])
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+  @@index([tenantId, date])
+  @@index([referenceType, referenceId])
+}
+
+/// A single line (debit or credit) within a JournalEntry.
+/// Every entry must have at least 2 lines, and sum(debit) = sum(credit).
+model JournalEntryLine {
+  id             String       @id @default(uuid())
+  tenantId       String
+  journalEntryId String
+  journalEntry   JournalEntry @relation(fields: [journalEntryId], references: [id])
+  accountId      String
+  account        Account      @relation(fields: [accountId], references: [id])
+  /// Debit amount for this line (0 if this is a credit line)
+  debit          Decimal      @default(0) @db.Decimal(15, 2)
+  /// Credit amount for this line (0 if this is a debit line)
+  credit         Decimal      @default(0) @db.Decimal(15, 2)
+  /// Optional line-level memo
+  description    String?
+  createdAt      DateTime     @default(now())
+
+  /// RLS-ready index — always present on tenant-scoped models
+  @@index([tenantId])
+  @@index([accountId])
+  @@index([journalEntryId])
+}