POST /api/packets INSERTs on the server's read-only handle and always returns 500 #223

Relates to #215, #222

Problem

handlePostPacket (cmd/server/routes.go, POST /api/packets, behind requireAPIKey) writes transmissions, observers and observations through s.db.conn. The server opens that handle with mode=ro (Kpa-clawbot#1283).

• In production every call fails with attempt to write a readonly database and returns 500, with the raw SQLite error text in the response.
• Route tests pass only because they run against a writable test DB.
• The handler also inserts without a by-hash lookup, so a repeated packet would violate transmissions.hash UNIQUE even on a writable DB.

This was confirmed in the #222 review with a probe: OpenDB (mode=ro) plus a POST with a valid key returns 500, and the DB stays unchanged. In practice the endpoint has been dead since the read/write split.

Options

• (a) Hand the packet to the ingestor so the write happens where all writes live, e.g. via a queue or command table, or by republishing it over MQTT.
• (b) Remove the endpoint and its OpenAPI entry, if nothing uses it.

In both cases:

• remove routes.go from knownServerWriteSQL;
• extend TestServerHasNoPacketTableWrites to forbid INSERT everywhere in cmd/server;
• add a test that POSTs against an OpenDB (mode=ro) store.

Acceptance

• No write SQL left in cmd/server outside the documented exceptions (ping_score_history.go, backup.go).
• The endpoint either works through the ingestor or is removed, and the OpenAPI spec matches.
• A read-only-handle test pins the behaviour.
• No raw SQLite error text in API responses.