feat(app): save household planner prefs over the session

[frahlg]

Contract-pair: srcfl/ftw-webapp@cursor/plan-household-prefs-1a28

Problem and result

The phone app needs to save the Plan card's household prefs. GET /api/planner/prefs is already a read. POST /api/planner/prefs is actuation, so the session was refusing it with E_USE_CMD and no command to name.

planner.prefs.set stores safety_k and battery_export through the same write the HTTP route performs. The box maps that permission onto a planner mode and returns the stored factor. The caller does not choose the mode.

planner_passive_arbitrage and planner_arbitrage stay in the mode catalogue. The dashboard hides those keys; Home Assistant still uses them.

Paired with srcfl/ftw-webapp#76. contract/registry.yaml is the same file in both.

Scope and safety

The command uses ftw.mode.write and is not a dispatch write, same as changing the site mode: a sick meter must not lock the household out of holding more reserve. The plan that comes out of the prefs still meets the dispatch gate before anything moves.

Open PRs #1474, #1470 and #1337 also touch go/internal/api/api.go. This change adds Via(planner.prefs.set) on the existing prefs POST only.

Verification

go test ./internal/appproto/ ./internal/api/ ./cmd/ftw -count=1 -run 'TestRegistry|TestPlannerPrefs|TestDoesNotExist' passed, which compiles the uplink wiring and runs the prefs command tests.

No UI in this repository.

Checklist

• The change follows one scope: the session door for an actuation route that already exists.
• Overlapping PRs checked. Shared file is go/internal/api/api.go.
• Tests cover a missing port, bad arguments, and an applied write.
• No UI changed.
• Changeset included.
• Commit has a DCO sign-off.