feat(proxy): /api/current — tidal current + water temp (KHOA 7-day model)

Adds hourly current speed/direction, water temperature and salinity for any
coordinate, from the KHOA ocean-grid 7-day endpoint (forward-covering today
+3 days). Powers the backlog 조류세기 gauge and 수온 readout:
- adapters/khoaMarine.ts: buildMarineDay() (pure, filters the 168h response
  to the requested day, computes daily max current for the % gauge) +
  fetchMarineDay() (shares khoaModel's session cookie).
- routes/current.ts: /api/current?lat&lon&date, 1km grid + 6h cache; 502 on
  failure so the app degrades silently (optional feature).
- types.ts: MarineHour/MarineDay.

Verified live: 인천 6/5 -> 24 hours, maxSpeed 1.64 m/s, 08:00 0.44 m/s 북 19.05℃.

Constraint: unofficial KHOA endpoint — optional layer; route returns 502 (not a hard error) so the client just hides the section when unavailable.
Confidence: high
Scope-risk: module
Reversibility: trivial
Directive: Surface current as a %-of-daily-max gauge in the app; keep water temp labelled as model forecast, not observation.
Tested: vitest 26/26 (incl. buildMarineDay date filtering); live HTTP against deployed proxy.
Not-tested: current-direction accuracy vs official current-prediction stations; multi-day window edges.
Related: docs/BACKLOG.md 2/4

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