feat(memory): add DatabaseMemoryService with SQLAlchemy async backend

Adds a durable, SQL-backed memory service that works with any
SQLAlchemy-compatible async database (SQLite, PostgreSQL, MySQL/MariaDB).

This fills the gap between the volatile InMemoryMemoryService and
the cloud-only Firestore/Vertex AI options, giving self-hosted
deployments a persistent memory backend with zero cloud dependencies.

The implementation mirrors the keyword-extraction approach used by
FirestoreMemoryService and reuses the existing SQLAlchemy patterns
established by DatabaseSessionService.

Closes #2524
Closes #2976

Author: anmolg1997

src/google/adk/memory/__init__.py

@@ -21,10 +21,26 @@
 
 __all__ = [
     'BaseMemoryService',
+    'DatabaseMemoryService',
     'InMemoryMemoryService',
     'VertexAiMemoryBankService',
 ]
 
+
+def __getattr__(name: str):
+  if name == 'DatabaseMemoryService':
+    try:
+      from .database_memory_service import DatabaseMemoryService
+
+      return DatabaseMemoryService
+    except ImportError as e:
+      raise ImportError(
+          'DatabaseMemoryService requires sqlalchemy>=2.0, please ensure it is'
+          ' installed correctly.'
+      ) from e
+  raise AttributeError(f'module {__name__!r} has no attribute {name!r}')
+
+
 try:
   from .vertex_ai_rag_memory_service import VertexAiRagMemoryService
 

src/google/adk/memory/_memory_schemas.py

@@ -0,0 +1,65 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""SQLAlchemy ORM models for DatabaseMemoryService."""
+
+from __future__ import annotations
+
+from typing import Any
+from typing import Optional
+
+from sqlalchemy import Float
+from sqlalchemy import Index
+from sqlalchemy import Text
+from sqlalchemy.orm import DeclarativeBase
+from sqlalchemy.orm import Mapped
+from sqlalchemy.orm import mapped_column
+from sqlalchemy.types import String
+
+from ..sessions.schemas.shared import DynamicJSON
+
+_MAX_KEY_LENGTH = 128
+
+
+class Base(DeclarativeBase):
+  """Base class for memory database tables."""
+
+  pass
+
+
+class StorageMemoryEntry(Base):
+  """Represents a single memory entry stored in the database."""
+
+  __tablename__ = "adk_memory_entries"
+
+  id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
+
+  app_name: Mapped[str] = mapped_column(String(_MAX_KEY_LENGTH), index=True)
+  user_id: Mapped[str] = mapped_column(String(_MAX_KEY_LENGTH), index=True)
+
+  keywords: Mapped[str] = mapped_column(Text)
+
+  author: Mapped[Optional[str]] = mapped_column(
+      String(_MAX_KEY_LENGTH), nullable=True
+  )
+  content: Mapped[dict[str, Any]] = mapped_column(DynamicJSON)
+  timestamp: Mapped[Optional[float]] = mapped_column(Float, nullable=True)
+
+  __table_args__ = (Index("idx_memory_app_user", "app_name", "user_id"),)
+
+  def __repr__(self) -> str:
+    return (
+        f"<StorageMemoryEntry(id={self.id}, app_name={self.app_name!r}, "
+        f"user_id={self.user_id!r})>"
+    )