feat: Add database adapter interface for multi-backend support (Phase 2)

Implement the adapter pattern to abstract database-specific logic and enable
PostgreSQL support alongside MySQL. This is Phase 2 of the PostgreSQL support
implementation plan (POSTGRES_SUPPORT.md).

New modules:
- src/datajoint/adapters/base.py: DatabaseAdapter abstract base class defining
  the complete interface for database operations (connection management, SQL
  generation, type mapping, error translation, introspection)
- src/datajoint/adapters/mysql.py: MySQLAdapter implementation with extracted
  MySQL-specific logic (backtick quoting, ON DUPLICATE KEY UPDATE, SHOW
  commands, information_schema queries)
- src/datajoint/adapters/postgres.py: PostgreSQLAdapter implementation with
  PostgreSQL-specific SQL dialect (double-quote quoting, ON CONFLICT,
  INTERVAL syntax, enum type management)
- src/datajoint/adapters/__init__.py: Adapter registry with get_adapter()
  factory function

Dependencies:
- Added optional PostgreSQL dependency: psycopg2-binary>=2.9.0
  (install with: pip install 'datajoint[postgres]')

Tests:
- tests/unit/test_adapters.py: Comprehensive unit tests for both adapters
  (24 tests for MySQL, 21 tests for PostgreSQL when psycopg2 available)
- All tests pass or properly skip when dependencies unavailable
- Pre-commit hooks pass (ruff, mypy, codespell)

Key features:
- Complete abstraction of database-specific SQL generation
- Type mapping between DataJoint core types and backend SQL types
- Error translation from backend errors to DataJoint exceptions
- Introspection query generation for schema, tables, columns, keys
- PostgreSQL enum type lifecycle management (CREATE TYPE/DROP TYPE)
- No changes to existing DataJoint code (adapters are standalone)

Phase 2 Status: ✅ Complete
Next phases: Configuration updates, connection refactoring, SQL generation
integration, testing with actual databases.

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

Author: dimitri-yatsenko

pyproject.toml

@@ -98,6 +98,7 @@ test = [
 s3 = ["s3fs>=2023.1.0"]
 gcs = ["gcsfs>=2023.1.0"]
 azure = ["adlfs>=2023.1.0"]
+postgres = ["psycopg2-binary>=2.9.0"]
 polars = ["polars>=0.20.0"]
 arrow = ["pyarrow>=14.0.0"]
 test = [

src/datajoint/adapters/__init__.py

@@ -0,0 +1,54 @@
+"""
+Database adapter registry for DataJoint.
+
+This module provides the adapter factory function and exports all adapters.
+"""
+
+from __future__ import annotations
+
+from .base import DatabaseAdapter
+from .mysql import MySQLAdapter
+from .postgres import PostgreSQLAdapter
+
+__all__ = ["DatabaseAdapter", "MySQLAdapter", "PostgreSQLAdapter", "get_adapter"]
+
+# Adapter registry mapping backend names to adapter classes
+ADAPTERS: dict[str, type[DatabaseAdapter]] = {
+    "mysql": MySQLAdapter,
+    "postgresql": PostgreSQLAdapter,
+    "postgres": PostgreSQLAdapter,  # Alias for postgresql
+}
+
+
+def get_adapter(backend: str) -> DatabaseAdapter:
+    """
+    Get adapter instance for the specified database backend.
+
+    Parameters
+    ----------
+    backend : str
+        Backend name: 'mysql', 'postgresql', or 'postgres'.
+
+    Returns
+    -------
+    DatabaseAdapter
+        Adapter instance for the specified backend.
+
+    Raises
+    ------
+    ValueError
+        If the backend is not supported.
+
+    Examples
+    --------
+    >>> from datajoint.adapters import get_adapter
+    >>> mysql_adapter = get_adapter('mysql')
+    >>> postgres_adapter = get_adapter('postgresql')
+    """
+    backend_lower = backend.lower()
+
+    if backend_lower not in ADAPTERS:
+        supported = sorted(set(ADAPTERS.keys()))
+        raise ValueError(f"Unknown database backend: {backend}. " f"Supported backends: {', '.join(supported)}")
+
+    return ADAPTERS[backend_lower]()