Group stats by source kind and add rolling event counts per station

[bkeepers]

/v1/stats listed every API client, UDP sender and HTTP feeder as its own source, so our own feeds showed up as a dozen v1:ed25519:… and udp:… entries next to each upstream. They now roll up into one entry per kind: v1, udp, http, mmsi, with the upstreams keeping their names. Event windows are summed per kind, vessels and vessels_exclusive are unioned per kind, and last_age_s is the youngest member.

Each row in /v1/stations now carries events as {"last_24h", "last_7d"} from the same 7-day hourly ring that backs /v1/stats. That ring replaces the 24-hour one the earned feeder tier used, so the tier reads the same numbers. Station rings are saved in the usage file and re-attached when a station reports again after a restart, so a deploy neither zeroes a feeder's counts nor lists stations that haven't reported since. duplicates and positions remain since first heard.

The viewer's station panel shows the 24-hour count; docs/API.md describes both shapes. Tests cover the per-kind grouping, a source that only ever duplicates, and the station-ring restore.

[Copilot]

Pull request overview

This PR updates AISCast’s usage and station statistics so /v1/stats and /v1/stations present more actionable aggregates: sources are rolled up by “source kind” (e.g., v1, udp, http, mmsi, plus upstream names), and stations now expose rolling event-window counts derived from the same 7‑day hourly ring used for stats and persisted across restarts.

Changes:

• Roll up /v1/stats.sources by source kind, unioning vessel sets and summing event windows per kind.
• Replace per-station 24h buckets with the shared 7‑day hourly ring and expose events: {last_24h, last_7d} on /v1/stations (persisted in the usage file and restored on restart).
• Update viewer and API docs/tests to match the new events shape.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
viewer/index.html	Updates station panel to display events.last_24h.
server/stats.go	Persists/restores station rings; groups stats sources by kind; adjusts stats payload shapes.
server/stats_test.go	Updates per-kind expectations and adds restart/restore test for station rings.
server/stations.go	Introduces 7‑day hourRing for station events, adds ring persistence/restore, and groups vessel counts by source kind.
server/pipeline.go	Records usage events per source kind (via sourceKind).
server/hub_test.go	Updates station stats assertions to new events map shape.
docs/API.md	Updates examples and endpoint documentation for the new events and per-kind sources shapes.

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