[FEATURE] Ship tracking map layer (AIS feed)

[accius]

Is your feature request related to a problem?

Spun off from #996 — the airplane tracking half of that issue shipped, but ship tracking was deferred because it needs an API key (aisstream.io, MarineTraffic, etc.) which is a separate deploy-config conversation.

Hams who'd find ship tracking useful: anyone working /MM stations, monitoring marine HF nets, supporting maritime mobile ops, supporting events where marine traffic matters (coastal Field Day, Coast Guard support, search-and-rescue exercises).

Describe the Solution You'd Like

A new toggleable map layer with the same server-cache + share-across-users pattern aircraft uses (see server/routes/aircraft.js for the template):

• Server route GET /api/ships proxying an AIS feed, with a ~30 s cache + stale-on-error fallback + in-flight refresh deduplication
• Client layer src/plugins/layers/useShips.js with metadata + useLayer export, registered in layerRegistry.js
• Markers oriented by ship heading, color-coded by AIS ship type (cargo / tanker / passenger / fishing / pleasure / military / other)
• Popup with name, MMSI, type, speed, destination, last-seen

Feed candidates:

• aisstream.io — free tier with API key; WebSocket push (would need to be terminated server-side and rebroadcast over our REST endpoint)
• MarineTraffic API — paid tiers, regional free options
• AIS Hub — peer-network, requires contributing your own AIS receiver data to access it (not viable for a public install)

Key bit: the layer should be hidden (localOnly or env-gated) if no key is configured, so installs without a key still work. Don't ship a half-functional toggle.

Use Case

• General HF operation
• VHF/UHF operation
• Satellite operation
• Contesting
• POTA/SOTA
• DXing (/MM stations on rare DXCC entities)
• Emergency communications (situational awareness, Coast Guard exercises)
• Other: maritime mobile ops, coastal special events, marine HF nets

Would you be willing to help implement this?

• Yes, I can submit a PR
• Yes, I can help test
• No, but I'd use it!

Additional Context

Aircraft tracking from #996 landed in d95908b and is the template — both server and client patterns are already in place. The main novel work for ships is the feed choice + key handling + a more nuanced color/icon scheme for the AIS ship-type field.