feat: add 30-day egg stats computation service

Implement compute-on-read egg statistics per spec section 9:
- Create egg_stats_30d_by_location table (migration 0007)
- Add get_egg_stats service with bird-days calculation
- Calculate layer-eligible days (adult female + matching species)
- Implement feed proration formula with INTEGER truncation
- Cache computed stats with window bounds

Verifies E2E test #1 baseline values:
- eggs_total_pcs = 12
- feed_total_g = 6000, feed_layers_g = 4615
- cost_per_egg_all = 0.600, cost_per_egg_layers = 0.462
- layer_eligible_bird_days = 10

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/0007-egg-stats-30d.sql

@@ -0,0 +1,22 @@
+-- ABOUTME: Migration to create egg_stats_30d_by_location table.
+-- ABOUTME: Caches computed 30-day rolling statistics for egg production.
+
+-- Egg stats computed on-read and cached per location
+-- Feed amounts in grams (not kg) for precision
+-- Costs stored as REAL EUR values
+CREATE TABLE egg_stats_30d_by_location (
+    location_id TEXT PRIMARY KEY REFERENCES locations(id),
+    window_start_utc INTEGER NOT NULL,
+    window_end_utc INTEGER NOT NULL,
+    eggs_total_pcs INTEGER NOT NULL,
+    feed_total_g INTEGER NOT NULL,
+    feed_layers_g INTEGER NOT NULL,
+    cost_per_egg_all_eur REAL NOT NULL,
+    cost_per_egg_layers_eur REAL NOT NULL,
+    layer_eligible_bird_days INTEGER NOT NULL,
+    layer_eligible_count_now INTEGER NOT NULL,
+    updated_at_utc INTEGER NOT NULL
+);
+
+-- Index for finding stale stats that need recomputation
+CREATE INDEX idx_egg_stats_updated ON egg_stats_30d_by_location(updated_at_utc);