fix: PostgreSQL compatibility for singleton tables

Add boolean_true_literal adapter property to generate correct
DEFAULT value for the hidden _singleton attribute:
- MySQL: DEFAULT 1 (bool maps to tinyint)
- PostgreSQL: DEFAULT TRUE (native boolean)

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

Author: dimitri-yatsenko

src/datajoint/adapters/base.py

@@ -563,6 +563,21 @@ def supports_inline_indexes(self) -> bool:
         """
         return True  # Default for MySQL, override in PostgreSQL
 
+    @property
+    def boolean_true_literal(self) -> str:
+        """
+        Return the SQL literal for boolean TRUE.
+
+        MySQL uses 1 (since bool maps to tinyint).
+        PostgreSQL uses TRUE (native boolean type).
+
+        Returns
+        -------
+        str
+            SQL literal for boolean true value.
+        """
+        return "1"  # Default for MySQL, override in PostgreSQL
+
     def create_index_ddl(
         self,
         full_table_name: str,

src/datajoint/adapters/postgres.py

@@ -710,6 +710,20 @@ def supports_inline_indexes(self) -> bool:
         """
         return False
 
+    @property
+    def boolean_true_literal(self) -> str:
+        """
+        Return the SQL literal for boolean TRUE.
+
+        PostgreSQL uses native boolean type with TRUE literal.
+
+        Returns
+        -------
+        str
+            SQL literal for boolean true value.
+        """
+        return "TRUE"
+
     # =========================================================================
     # Introspection
     # =========================================================================

src/datajoint/declare.py

@@ -477,11 +477,12 @@ def declare(
         primary_key = ["_singleton"]
         singleton_comment = ":bool:singleton primary key"
         sql_type = adapter.core_type_to_sql("bool")
+        bool_literal = adapter.boolean_true_literal
         singleton_sql = adapter.format_column_definition(
             name="_singleton",
             sql_type=sql_type,
             nullable=False,
-            default="NOT NULL DEFAULT 1",
+            default=f"NOT NULL DEFAULT {bool_literal}",
             comment=singleton_comment,
         )
         attribute_sql.insert(0, singleton_sql)