source-coop api: raw requests to /api/v1 #27

source-coop api <method> <path> [-f key=value] [--input file], the gh api equivalent. It works with any endpoint as soon as the endpoint ships, before a dedicated command exists. Build this first.

• Authenticate with the existing cached login, or a service account API key (feat: exchange a service account's API key without a browser #20), sent as Authorization: Bearer. The other command groups reuse this request path.
• -f key=value builds a JSON body; --input file (or -) sends a file as the body.
• Print the response body; exit non-zero on a 4xx/5xx and show field_errors.
• Needs only the existing API.

Part of #21. Per the epic, the CLI validates nothing itself: it sends the request and shows the API's field_errors. Add a CLI path to docs.source.coop docs/using-source/ wherever this flow gets one.