docs: backlog — current/water-temp done, add breakwater-search follow-up

- §2 조류세기 and §4 수온 marked done (v0.4.2, via /api/current).
- New §0: breakwater/named-port search via Kakao Local keyword API — blocked
  on the kakago app's disabled OPEN_MAP_AND_LOCAL service; documents the user
  action (enable the service or issue a Sea&Catch REST key) and the proxy
  /api/place route that would hide the key and reuse the coordinate tide path.

Constraint: doc-only.
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: Once a Kakao Local key is available, add proxy /api/place and wire TopSearch results to the coordinate tide/current path.
Tested: N/A (doc).
Not-tested: N/A
Related: docs/BACKLOG.md

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