perf: implement batch operations and optimize database indexes

Add batch upsert functionality to reduce database round-trips from N to 1 when inserting courses. Create comprehensive database indexes for common query patterns including term/subject lookups, time-series metrics, and job scheduling. Remove redundant indexes and add monitoring guidance for BRIN index effectiveness.

migrations/20251103104400_optimize_indexes.sql

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+-- Index Optimization Follow-up Migration
+
+-- Reason: Redundant with composite index idx_courses_subject_term
+DROP INDEX IF EXISTS idx_courses_subject;
+
+-- Remove: idx_scrape_jobs_retry_count
+DROP INDEX IF EXISTS idx_scrape_jobs_retry_count;
+
+-- Purpose: Optimize the scheduler's frequent query (runs every 60 seconds)
+CREATE INDEX IF NOT EXISTS idx_scrape_jobs_scheduler_lookup
+    ON scrape_jobs(target_type, target_payload)
+    WHERE locked_at IS NULL;
+
+-- Note: We use (target_type, target_payload) instead of including locked_at
+-- in the index columns because:
+-- 1. The WHERE clause filters locked_at IS NULL (partial index optimization)
+-- 2. target_payload is JSONB and already large; keeping it as an indexed column
+--    allows PostgreSQL to use index-only scans for the SELECT target_payload query
+-- 3. This design minimizes index size while maximizing query performance
+
+
+-- Purpose: Enable efficient audit trail queries by course
+CREATE INDEX IF NOT EXISTS idx_course_audits_course_timestamp
+    ON course_audits(course_id, timestamp DESC);
+
+-- Purpose: Enable queries like "Show all changes in the last 24 hours"
+CREATE INDEX IF NOT EXISTS idx_course_audits_timestamp
+    ON course_audits(timestamp DESC);
+
+
+-- The BRIN index on course_metrics(timestamp) assumes data is inserted in
+-- chronological order. BRIN indexes are only effective when data is physically
+-- ordered on disk. If you perform:
+-- - Backfills of historical data
+-- - Out-of-order inserts
+-- - Frequent UPDATEs that move rows
+--
+-- Then the BRIN index effectiveness will degrade. Monitor with:
+--   SELECT * FROM brin_page_items(get_raw_page('idx_course_metrics_timestamp', 1));
+--
+-- If you see poor selectivity, consider:
+-- 1. REINDEX to rebuild after bulk loads
+-- 2. Switch to B-tree if inserts are not time-ordered
+-- 3. Use CLUSTER to physically reorder the table (requires downtime)
+
+COMMENT ON INDEX idx_course_metrics_timestamp IS
+    'BRIN index - requires chronologically ordered inserts for efficiency. Monitor selectivity.';
+
+-- Update statistics for query planner
+ANALYZE courses;
+ANALYZE course_metrics;
+ANALYZE course_audits;
+ANALYZE scrape_jobs;