refactor(#1512): express FK auto-indexing as an adapter capability

Replace the `adapter.backend == "postgresql"` string check in declare.py with a
new adapter capability, `auto_indexes_foreign_keys` (MySQL True, PostgreSQL
False), mirroring `supports_inline_indexes`. The backend *policy* (who indexes FK
columns) now lives on the adapter; the coverage/redundancy logic stays in
declare.py as backend-agnostic schema-structure reasoning. Scales to new backends
without touching declare.py.

Author: dimitri-yatsenko

src/datajoint/adapters/base.py

@@ -617,6 +617,22 @@ def supports_inline_indexes(self) -> bool:
         """
         return True  # Default for MySQL, override in PostgreSQL
 
+    @property
+    def auto_indexes_foreign_keys(self) -> bool:
+        """
+        Whether this backend implicitly indexes a foreign key's referencing
+        (child) columns as part of enforcing the constraint.
+
+        MySQL/InnoDB does; PostgreSQL does not (and offers no server setting to
+        make it), so DataJoint must emit an explicit index on that backend.
+
+        Returns
+        -------
+        bool
+            True for MySQL, False for PostgreSQL.
+        """
+        return True  # Default for MySQL, override in PostgreSQL
+
     def create_index_ddl(
         self,
         full_table_name: str,

src/datajoint/adapters/postgres.py

@@ -720,6 +720,14 @@ def supports_inline_indexes(self) -> bool:
         """
         return False
 
+    @property
+    def auto_indexes_foreign_keys(self) -> bool:
+        """
+        PostgreSQL never indexes a foreign key's referencing columns
+        automatically, so DataJoint emits an explicit (coverage-aware) index.
+        """
+        return False
+
     # =========================================================================
     # Introspection
     # =========================================================================

src/datajoint/declare.py

@@ -321,7 +321,7 @@ def compile_foreign_key(
     if is_unique:
         index_cols = ", ".join(adapter.quote_identifier(attr) for attr in fk_attrs)
         index_sql.append(f"UNIQUE INDEX ({index_cols})")
-    elif adapter.backend == "postgresql" and fk_index_candidates is not None:
+    elif not adapter.auto_indexes_foreign_keys and fk_index_candidates is not None:
         fk_index_candidates.append(fk_attrs)