Based on the repository's commit message style and the changes in the diff, here's an appropriate commit message:

```
feat: add websocket tests, PoA metrics, marketplace endpoints, and enhanced observability

- Add comprehensive websocket tests for blocks and transactions streams including multi-subscriber and high-volume scenarios
- Extend PoA consensus with per-proposer block metrics and rotation tracking
- Add latest block interval gauge and RPC error spike alerting
- Enhance mock coordinator

apps/coordinator-api/src/app/domain/__init__.py

@@ -3,5 +3,13 @@
 from .job import Job
 from .miner import Miner
 from .job_receipt import JobReceipt
+from .marketplace import MarketplaceOffer, MarketplaceBid, OfferStatus
 
-__all__ = ["Job", "Miner", "JobReceipt"]
+__all__ = [
+    "Job",
+    "Miner",
+    "JobReceipt",
+    "MarketplaceOffer",
+    "MarketplaceBid",
+    "OfferStatus",
+]

apps/coordinator-api/src/app/domain/marketplace.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+from datetime import datetime
+from enum import Enum
+from typing import Optional
+from uuid import uuid4
+
+from sqlalchemy import Column, Enum as SAEnum, JSON
+from sqlmodel import Field, SQLModel
+
+
+class OfferStatus(str, Enum):
+    open = "open"
+    reserved = "reserved"
+    closed = "closed"
+
+
+class MarketplaceOffer(SQLModel, table=True):
+    id: str = Field(default_factory=lambda: uuid4().hex, primary_key=True)
+    provider: str = Field(index=True)
+    capacity: int = Field(default=0, nullable=False)
+    price: float = Field(default=0.0, nullable=False)
+    sla: str = Field(default="")
+    status: OfferStatus = Field(default=OfferStatus.open, sa_column=Column(SAEnum(OfferStatus), nullable=False))
+    created_at: datetime = Field(default_factory=datetime.utcnow, nullable=False, index=True)
+    attributes: dict = Field(default_factory=dict, sa_column=Column(JSON, nullable=False))
+
+
+class MarketplaceBid(SQLModel, table=True):
+    id: str = Field(default_factory=lambda: uuid4().hex, primary_key=True)
+    provider: str = Field(index=True)
+    capacity: int = Field(default=0, nullable=False)
+    price: float = Field(default=0.0, nullable=False)
+    notes: Optional[str] = Field(default=None)
+    status: str = Field(default="pending", nullable=False)
+    submitted_at: datetime = Field(default_factory=datetime.utcnow, nullable=False, index=True)