Give connectors a store that survives restarts, and a way to clear it

`ConnectorNode.store` keeps short strings per connector package in the
engine's database (`connector_state`, migration f8c3e5a91d27), so the nodes of
one package share them and `fluksio serve` needs nothing new: get, set and
delete, at most 64 kB a value and 256 keys a package. A superuser lists the
keys (never the values) and clears them through `/connectors/{node_type}/state`,
or under Stored state in a connector node's panel. Documented in the connector
contract.

backend/fluksio/api/main.py

@@ -5,6 +5,7 @@ from fluksio.api.routes import (
     alerts,
     artifacts,
     cloud,
+    connectors,
     dashboards,
     flavors,
     flows,
@@ -31,6 +32,7 @@ api_router.include_router(flavors.router)
 api_router.include_router(flows.router)
 api_router.include_router(flows.ws_router)
 api_router.include_router(secrets.router)
+api_router.include_router(connectors.router)
 api_router.include_router(alerts.router)
 api_router.include_router(dashboards.router)
 api_router.include_router(panels.router)

backend/fluksio/api/routes/connectors.py

@@ -0,0 +1,79 @@
+"""What a connector keeps across restarts, for a superuser to see and reset.
+
+A connector writes these itself (``ConnectorNode.store``). This is the way back
+when one of them is wrong: a Hue bridge replaced, so the certificate pinned for
+it no longer matches. Values are never handed out, since a connector may keep a
+token here; the keys say enough to decide what to clear.
+
+A connector is named by any node type it installs, as the canvas knows it, and
+its keys are shared by every node type of the same package.
+"""
+
+from datetime import datetime
+from typing import Any
+
+from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
+
+from fluksio.api.deps import get_current_active_superuser
+from fluksio.flow.connector import ConnectorNode
+from fluksio.flow.connector_store import KEY, ConnectorStore, namespace_of
+from fluksio.flow.controller import NODE_TYPES
+from fluksio.models import Message
+
+router = APIRouter(
+    prefix="/connectors",
+    tags=["connectors"],
+    dependencies=[Depends(get_current_active_superuser)],
+)
+
+
+class StoredKey(BaseModel):
+    key: str
+    #: Bytes, so a superuser can tell a fingerprint from a blob.
+    size: int
+    updated_at: datetime
+
+
+class ConnectorStatePublic(BaseModel):
+    #: The package the keys belong to.
+    connector: str
+    data: list[StoredKey]
+    count: int
+
+
+def _store(node_type: str) -> ConnectorStore:
+    """The store of the connector that installed *node_type*, or a 404."""
+    known = NODE_TYPES.get(node_type)
+    cls = known.cls if known is not None else None
+    if not (isinstance(cls, type) and issubclass(cls, ConnectorNode)):
+        raise HTTPException(status_code=404, detail=f"No connector named '{node_type}'")
+    return ConnectorStore(namespace_of(cls))
+
+
+@router.get("/{node_type}/state", response_model=ConnectorStatePublic)
+def read_connector_state(node_type: str) -> Any:
+    """The keys this connector keeps, without their values."""
+    store = _store(node_type)
+    keys = [
+        StoredKey(key=row.key, size=len(row.value.encode()), updated_at=row.updated_at)
+        for row in store.entries()
+    ]
+    return ConnectorStatePublic(connector=store.connector, data=keys, count=len(keys))
+
+
+@router.delete("/{node_type}/state", response_model=Message)
+def clear_connector_state(node_type: str) -> Any:
+    """Forget everything this connector keeps. It starts over on next use."""
+    store = _store(node_type)
+    cleared = store.clear()
+    return Message(message=f"Cleared {cleared} key(s) of {store.connector}")
+
+
+@router.delete("/{node_type}/state/{key}", response_model=Message)
+def delete_connector_state(node_type: str, key: str) -> Any:
+    """Forget one key, such as the certificate pinned for one bridge."""
+    store = _store(node_type)
+    if not KEY.fullmatch(key) or not store.delete(key):
+        raise HTTPException(status_code=404, detail=f"No key '{key}' stored")
+    return Message(message=f"Deleted '{key}' of {store.connector}")