feat: Prisma @relation field support with FK generation (Issue #10)

[Phathdt]

Summary

Implements full support for Prisma @relation fields, enabling FK constraint generation in migrations.

Changes

Core Implementation

• Full @relation parsing: fields, references, onDelete, onUpdate
• FK generation: Create schema.ForeignKey from @relation attributes
• Composite FK support: Multi-column foreign keys
• Field/model mapping: Prisma names → DB names (respect @Map, @@Map)
• Three-pass conversion:
  1. Collect metadata (enums, model→table, field→column maps)
  2. Convert models to internal schema
  3. Validate FK references exist in target tables

Differ Enhancement

• Add CreateForeignKey operations for initial migrations

Test Coverage (+11 tests)

Integration Tests (7)

• Simple relation → FK generation
• Composite FK (multi-column)
• Referential actions (Cascade, Restrict, SetNull, SetDefault, NoAction)
• Nullable relations
• Back-reference handling
• Error cases

E2E Pipeline Tests (3)

• Prisma → Parse → Diff → SQL with FK DDL verification
• Composite FK SQL generation
• Multiple independent FKs

Manual Test (1)

• Real-world schema verification

Example Output

ALTER TABLE posts
ADD CONSTRAINT fk_posts_author_id
FOREIGN KEY (author_id)
REFERENCES users (id) ON DELETE CASCADE;

Verification

✅ All 68 integration tests pass
✅ FK names use actual table names
✅ RefColumns correctly mapped
✅ Handles forward references

Closes #10