feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/crm.dbml

@@ -0,0 +1,53 @@
+Table professional_contacts {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  company text
+  title text
+  email text
+  phone text
+  linkedin_url text
+  how_we_met text
+  tags "text[]" [not null, default: `'{}'`]
+  notes text
+  last_contacted timestamptz
+  follow_up_date date
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    last_contacted
+    follow_up_date
+  }
+}
+
+Table contact_interactions {
+  id uuid [pk, default: `gen_random_uuid()`]
+  contact_id uuid [not null, ref: > professional_contacts.id]
+  interaction_type text [not null]
+  occurred_at timestamptz [not null, default: `now()`]
+  summary text [not null]
+  follow_up_needed boolean [not null, default: false]
+  follow_up_notes text
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    (contact_id, occurred_at)
+  }
+}
+
+Table opportunities {
+  id uuid [pk, default: `gen_random_uuid()`]
+  contact_id uuid [ref: > professional_contacts.id]
+  title text [not null]
+  description text
+  stage text [not null, default: 'identified']
+  value "decimal(12,2)"
+  expected_close_date date
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    stage
+  }
+}