fix(#551): trend labels, dedup persistence, recommendation unification

[TomikeDS]

Closes #551

Changes

Trend labels fixed

• RollingAverages time windows: short_term=5min, medium_term=1hr, long_term=24hr
• TrendChanges serde labels: 5m_pct, 1h_pct, 24h_pct (was 1h_pct, 6h_pct)

Congestion double-counting removed

• recommend() no longer applies network_condition_adjustment() - the base_fee percentile already reflects congestion

GET /fees/recommend consolidated with POST

• get_recommend handler now delegates to engine.recommend() with default params instead of its own divergent percentile logic

Recommendation history wired to persistence

• FeeRecommendationEngine accepts Arc via with_repository() builder
• recommend() persists via insert_recommendation() after computing
• GET /fees/recommend/history calls query_recent_recommendations(50)

Alert dedup made durable

• AlertManager gained rehydrate_seen_spikes() - loads from alert_events table at startup
• All dispatches (success/failure) are recorded in alert_events

Alert config CRUD connected to dispatcher

• check_and_dispatch() loads enabled configs from DB
• Dispatches to every enabled webhook whose threshold the spike meets
• Falls back to legacy single-webhook mode if no DB configs exist

Tests

• Added deterministic tests for: congestion double-counting prevention, target_ledgers fee ordering, trend label serde, percent_change calculation
• 205 passed, 1 failed (pre-existing upstream etag_is_quoted)
• Clippy clean, format clean