fix: blank company charts + competitor GUIDs instead of tickers

Trend charts blank:
- trend_windows uses upsert (1 row per ticker/window), so charts had
  at most 1 data point. Added trend_history table (migration 024) that
  appends every snapshot. New /api/trends/history endpoint serves the
  time series. Frontend now uses useTrendHistory for charts and
  useTrends for the latest summary card.

Competitor GUIDs:
- list_competitors query returned raw company_b_id UUIDs without
  joining companies table. Added LEFT JOIN with CASE to resolve the
  other company's ticker and legal_name. Updated Pydantic model to
  include enriched fields. Frontend fallback changed from truncated
  UUID to ticker/legal_name/Unknown.

infra/migrations/024_trend_history.sql

@@ -0,0 +1,37 @@
+-- Trend history table for time-series charting.
+-- trend_windows stores the latest snapshot per (entity, window) via upsert.
+-- trend_history stores every snapshot so the frontend can plot trend evolution.
+
+CREATE TABLE IF NOT EXISTS trend_history (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    entity_type VARCHAR(50) NOT NULL DEFAULT 'company',
+    entity_id VARCHAR(100) NOT NULL,
+    "window" VARCHAR(20) NOT NULL,
+    trend_direction VARCHAR(20) NOT NULL DEFAULT 'neutral',
+    trend_strength FLOAT DEFAULT 0.5,
+    confidence FLOAT DEFAULT 0.5,
+    contradiction_score FLOAT DEFAULT 0.0,
+    dominant_catalysts JSONB DEFAULT '[]',
+    material_risks JSONB DEFAULT '[]',
+    generated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_trend_history_lookup
+    ON trend_history (entity_id, "window", generated_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_trend_history_generated
+    ON trend_history (generated_at DESC);
+
+-- Seed history from existing trend_windows so charts aren't empty
+-- on first deploy. This gives at least one data point per ticker/window.
+INSERT INTO trend_history (
+    entity_type, entity_id, "window", trend_direction,
+    trend_strength, confidence, contradiction_score,
+    dominant_catalysts, material_risks, generated_at
+)
+SELECT
+    entity_type, entity_id, "window", trend_direction,
+    trend_strength, confidence, contradiction_score,
+    dominant_catalysts, material_risks, generated_at
+FROM trend_windows
+ON CONFLICT DO NOTHING;