perf: Add performance indexes to spans, agents, and tasks tables

[smoreinis]

Summary

• Add indexes to SpanORM: trace_id, (trace_id, start_time), parent_id
• Add index to AgentORM: status
• Add index to TaskORM: status

Problem

SpanORM had no indexes at all, causing full table scans on every query. AgentORM and TaskORM status columns were also unindexed despite being used in WHERE clauses.

Test Results

Metric	Without Index	With Index	Improvement
Query Plan	Seq Scan	Bitmap Index Scan	Direct lookup
Execution Time	0.185 ms	0.064 ms	~3x faster
Rows Scanned	894 (all)	2 (matches only)	99.8% reduction

Note: With larger datasets (100K+ rows), improvement would be 100x or more due to O(log N) vs O(N) complexity.

How to Verify

-- Check indexes exist
\di ix_spans_*
\di ix_agents_status
\di ix_tasks_status

-- Test query plan
EXPLAIN ANALYZE SELECT * FROM spans WHERE trace_id = 'xxx' ORDER BY start_time;
-- Should show "Bitmap Index Scan on ix_spans_trace_id_start_time"

Files Changed

• src/adapters/orm.py - Added __table_args__ with indexes
• database/migrations/alembic/versions/2025_12_24_*_add_performance_indexes.py - Migration