feat(agent): 检索工具、模型接入与结构化回复

Agent 按组织与知识源范围调用检索工具，密钥加密保存，回复含来源与转人工意图。

backend/app/services/usage_metering.py

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+"""
+Copyright 2024-2026 ChatterMate
+
+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.
+
+Shared usage-metering helpers for hosted-model features (FAQ generation, AI
+ticket investigations). Whether a run counts against the org's message budget,
+and how many credits remain, is the same question for every hosted-model
+feature — kept here so each feature doesn't reimplement (or drift from) it.
+
+OSS/self-hosted builds have no enterprise module: nothing is metered and
+nothing is blocked (fail open).
+"""
+
+from typing import Optional
+from uuid import UUID
+
+from sqlalchemy.orm import Session
+
+from app.core.logger import get_logger
+from app.repositories.ai_config import AIConfigRepository
+
+logger = get_logger(__name__)
+
+# Model type whose provider costs land on the platform, hence metered. Own-key
+# orgs pay their provider directly and are never metered or blocked.
+HOSTED_MODEL_TYPE = "CHATTERMATE"
+
+
+def is_hosted_model(db: Session, organization_id: UUID) -> bool:
+    """True when the org runs on the platform-hosted model, so its LLM usage is
+    billable against the message budget."""
+    config = AIConfigRepository(db).get_active_config(organization_id)
+    if not config:
+        return False
+    model_type = (
+        config.model_type.value if hasattr(config.model_type, "value")
+        else str(config.model_type)
+    )
+    return model_type.upper() == HOSTED_MODEL_TYPE
+
+
+def remaining_message_credits(db: Session, organization_id: UUID) -> Optional[int]:
+    """Message credits left in the current billing period, or None when
+    unlimited (no enterprise module, no subscription cap, or the check errored
+    — fail open, matching check_message_limit's posture)."""
+    try:
+        from app.enterprise.services.message_limit import get_remaining_messages
+    except ImportError:
+        return None
+    try:
+        return get_remaining_messages(db, organization_id)
+    except Exception as e:
+        logger.error(f"Remaining-credit check failed for org {organization_id}: {e}")
+        return None