build: 打通 FastAPI 与 Vue 最小工程、依赖与数据库连接

后端入口、配置、Redis 与 Alembic 迁移就位；前端 Vite 工程可安装可构建。

backend/alembic/versions/add_investigation_metered.py

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+"""Add metered flag to investigation_runs
+
+Revision ID: add_tkt_metered_001
+Revises: add_tkt_row_scope_001
+Create Date: 2026-07-21
+
+Marks whether an investigation run's LLM usage counts against the org's message
+budget (hosted model only). Existing rows default to false — nothing is
+retroactively metered on upgrade.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "add_tkt_metered_001"
+down_revision = "add_tkt_row_scope_001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "investigation_runs",
+        sa.Column(
+            "metered",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.text("false"),
+        ),
+    )
+    # The enterprise message-limit check sums llm_calls of metered runs per
+    # billing window — an index on (org, metered, created_at) keeps that cheap.
+    op.create_index(
+        "ix_investigation_runs_metered_period",
+        "investigation_runs",
+        ["organization_id", "metered", "created_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_investigation_runs_metered_period", table_name="investigation_runs")
+    op.drop_column("investigation_runs", "metered")