refactor(api): TOML 配置 SSOT、统一错误契约、Auth/事务加固与可观测性 (#33)

配置 SSOT（TOML + .env）
统一错误契约
Auth 与事务边界
Redis / Celery 可靠性:业务 Redis（DB/0）与 Celery broker/backend（DB/1）显式拆分；连接池、sync client
可观测性（OpenTelemetry + LGTM）

api/alembic/versions/0021_memory_source_segment_id.py

@@ -0,0 +1,83 @@
+"""memory_sources: segment_id for phase1 ingest idempotency
+
+Revision ID: 0021_memory_source_segment_id
+Revises: 0020_refresh_rt_lineage
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "0021_memory_source_segment_id"
+down_revision: Union[str, None] = "0020_refresh_rt_lineage"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def _column_names(table_name: str) -> set[str]:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    return {column["name"] for column in inspector.get_columns(table_name)}
+
+
+def _index_names(table_name: str) -> set[str]:
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    return {index["name"] for index in inspector.get_indexes(table_name)}
+
+
+def upgrade() -> None:
+    columns = _column_names("memory_sources")
+    if "segment_id" not in columns:
+        op.add_column(
+            "memory_sources",
+            sa.Column("segment_id", sa.String(), nullable=True),
+        )
+    indexes = _index_names("memory_sources")
+    if "ix_memory_sources_user_segment_transcript" not in indexes:
+        op.create_index(
+            "ix_memory_sources_user_segment_transcript",
+            "memory_sources",
+            ["user_id", "segment_id"],
+            unique=True,
+            postgresql_where=sa.text(
+                "segment_id IS NOT NULL AND source_type = 'transcript'"
+            ),
+        )
+    foreign_keys = {
+        fk["name"]
+        for fk in sa.inspect(op.get_bind()).get_foreign_keys("memory_sources")
+    }
+    if "fk_memory_sources_segment_id_segments" not in foreign_keys:
+        op.create_foreign_key(
+            "fk_memory_sources_segment_id_segments",
+            "memory_sources",
+            "segments",
+            ["segment_id"],
+            ["id"],
+            ondelete="SET NULL",
+        )
+
+
+def downgrade() -> None:
+    foreign_keys = {
+        fk["name"]
+        for fk in sa.inspect(op.get_bind()).get_foreign_keys("memory_sources")
+    }
+    if "fk_memory_sources_segment_id_segments" in foreign_keys:
+        op.drop_constraint(
+            "fk_memory_sources_segment_id_segments",
+            "memory_sources",
+            type_="foreignkey",
+        )
+    indexes = _index_names("memory_sources")
+    if "ix_memory_sources_user_segment_transcript" in indexes:
+        op.drop_index(
+            "ix_memory_sources_user_segment_transcript",
+            table_name="memory_sources",
+        )
+    columns = _column_names("memory_sources")
+    if "segment_id" in columns:
+        op.drop_column("memory_sources", "segment_id")