fix(P0-03): Prediction 幂等指纹——只追加,不覆盖

_upsert_prediction 改为 _insert_or_find_by_fingerprint:
- 同 input_hash → 返回已有行(绝不 UPDATE pred_/reasoning/agent_outputs)
- 不同 input_hash → INSERT 新行

input_hash 升级为规范 JSON SHA-256,捕获:match_id, cutoff, prompt_version,
prompt_hash, system_prompt_hash, provider, model, mode, run_type, temperature,
context_hash, agent_ids。移除旧 (match, provider, model, mode, run_type) 唯一约束,
改为 partial unique index(WHERE input_hash IS NOT NULL,兼容旧 NULL 数据)。

三条路径(single/multi/baseline)统一传足指纹字段。
迁移 0024 + 测试 test_p0_prediction_fingerprint(10/10);全量 295 通过。

alembic/versions/0024_prediction_idempotent_fingerprint.py

@@ -0,0 +1,41 @@
+"""P0-03: Prediction 幂等指纹——移除旧唯一约束,改为 partial unique on input_hash
+
+input_hash 非空时唯一(同指纹返回已有行,不 UPDATE/INSERT);
+兼容旧数据 NULL input_hash(不强制回填)。
+
+Revision ID: 0024_prediction_idempotent_fingerprint
+Revises: 0023_standings_append_only
+Create Date: 2026-09-22
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = '0024_prediction_idempotent_fingerprint'
+down_revision: Union[str, None] = '0023_standings_append_only'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # 移除旧唯一约束(match, provider, model, mode, run_type)
+    op.drop_constraint(
+        'uq_predictions_match_provider_model_mode_run_type',
+        'predictions', type_unique=True,
+    )
+    # P0-03: partial unique on input_hash(非空时唯一)
+    op.create_index(
+        'ix_predictions_input_hash_unique', 'predictions', ['input_hash'], unique=True,
+        postgresql_where=sa.text('input_hash IS NOT NULL'),
+    )
+
+
+def downgrade() -> None:
+    op.drop_index('ix_predictions_input_hash_unique', table_name='predictions')
+    op.create_unique_constraint(
+        'uq_predictions_match_provider_model_mode_run_type',
+        'predictions',
+        ['match_id', 'provider', 'model', 'mode', 'run_type'],
+    )