feat: add seasonal suggested keyword lookup endpoint

[hyunbin1]

Motivation

• Provide a simple API for the frontend to fetch 5 fixed seasonal recommendation keywords per period and fall back to realtime popular keywords during non-operational months (May, November).
• Make seasonal logic configurable by month so the frontend can request for a specific month or use the current month by default.

Description

• Added PopularSearchKeyword JPA entity with fields id, period (enum SeasonalPeriod), keyword, and displayOrder to store seasonal keywords.
• Added PopularSearchKeywordRepository with findTop5ByPeriodOrderByDisplayOrderAscIdAsc to fetch up to 5 keywords in display order.
• Implemented SeasonalSuggestService that resolves a month into seasonal buckets, validates month range, returns DB-based keywords for periods (개강/시험/종강/방학) and returns realtime top-5 via RealtimeKeywordService for off months (5, 11).
• Added SeasonalSuggestionResponse DTO and extended SuggestController with GET /api/v1/search/suggest/seasonal which accepts an optional month query parameter and returns { period, keywords }.

Testing

• Ran ./gradlew test --no-daemon but the build failed in this environment with a Java/Gradle compatibility error (Unsupported class file major version 69), so automated tests could not be executed here.

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