fix(0042_iam_tables.sql): Fix PostgreSQL partial unique constraint syntax

Issue: Table constraint with WHERE clause is invalid PostgreSQL syntax
Error: 42601: syntax error at or near "WHERE" at position 1525

Solution: Move partial uniqueness to separate CREATE UNIQUE INDEX statement
- Removed invalid WHERE clause from table UNIQUE() constraint
- Created proper partial unique index with WHERE condition
- PostgreSQL syntax now correct

Syntax fix:
  ❌ UNIQUE(col1, col2) WHERE condition  (invalid in table def)
  ✅ CREATE UNIQUE INDEX idx ON table(col1, col2) WHERE condition

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

db/migrations/0042_iam_tables.sql

@@ -108,10 +108,7 @@ CREATE TABLE IF NOT EXISTS public.role_assignment (
 
   -- Idempotency & correlation
   correlation_id UUID UNIQUE NOT NULL DEFAULT gen_random_uuid(),
-  checksum VARCHAR(64),
-
-  -- Constraints: One role per identity (except temporary/special cases)
-  UNIQUE(identity_id, role_id) WHERE assignment_state NOT IN ('REVOKED', 'REJECTED')
+  checksum VARCHAR(64)
 );
 
 CREATE INDEX idx_role_assignment_identity ON public.role_assignment(identity_id);
@@ -119,6 +116,11 @@ CREATE INDEX idx_role_assignment_role ON public.role_assignment(role_id);
 CREATE INDEX idx_role_assignment_state ON public.role_assignment(assignment_state);
 CREATE INDEX idx_role_assignment_correlation ON public.role_assignment(correlation_id);
 
+-- Partial unique constraint: One active role per identity
+CREATE UNIQUE INDEX idx_role_assignment_unique_active
+  ON public.role_assignment(identity_id, role_id)
+  WHERE assignment_state NOT IN ('REVOKED', 'REJECTED');
+
 -- 4. PERMISSION TABLE (Granular Permissions)
 CREATE TABLE IF NOT EXISTS public.permission (
   permission_id UUID PRIMARY KEY DEFAULT gen_random_uuid(),