Docs/installation tutorial (#7)

* [add]修改迁移文件新建空白表结构

* Add installation guide and environment setup

Added installation instructions and environment requirements for MemoryBear.

* [delete]删除api，web的readme.md。只保留唯一readme.md

* Fix database connection example in README

Update database connection configuration example in README.

api/migrations/versions/57c11f3c7aee_202511292027.py

@@ -20,11 +20,31 @@ depends_on: Union[str, Sequence[str], None] = None
 
 def upgrade() -> None:
     # ### commands auto generated by Alembic - please adjust! ###
-    op.add_column('data_config', sa.Column('llm', sa.String(), nullable=True, comment='LLM模型配置ID'))
+    # 检查表是否存在再添加列
+    op.execute("""
+        DO $$
+        BEGIN
+            IF EXISTS (SELECT FROM information_schema.tables WHERE table_name = 'data_config') 
+               AND NOT EXISTS (SELECT FROM information_schema.columns 
+                              WHERE table_name = 'data_config' AND column_name = 'llm') THEN
+                ALTER TABLE data_config ADD COLUMN llm VARCHAR;
+                COMMENT ON COLUMN data_config.llm IS 'LLM模型配置ID';
+            END IF;
+        END $$;
+    """)
     # ### end Alembic commands ###
 
 
 def downgrade() -> None:
     # ### commands auto generated by Alembic - please adjust! ###
-    op.drop_column('data_config', 'llm')
+    # 检查列是否存在再删除
+    op.execute("""
+        DO $$
+        BEGIN
+            IF EXISTS (SELECT FROM information_schema.columns 
+                      WHERE table_name = 'data_config' AND column_name = 'llm') THEN
+                ALTER TABLE data_config DROP COLUMN llm;
+            END IF;
+        END $$;
+    """)
     # ### end Alembic commands ###