feat: add multi-symbol Alpaca automation

[victorzhang674-crypto]

Summary

• add env-configured seven-symbol batch analysis with persistent 2-3 symbol rotation
• add conviction-weighted allocation capped at 30% of positive cash
• add safe Alpaca paper/live execution, position tracking, and 30-minute scheduling
• preserve the existing single-symbol graph and interactive CLI flow

Safety

• paper and dry-run remain defaults; live orders require explicit acknowledgment
• all seven decisions must be fresh before execution
• renewable SQLite leases prevent overlapping cron/foreground cycles
• deterministic persisted client order IDs prevent ambiguous-submit duplication
• closed equity markets and unsupported shorts fail closed

Verification

• 711 passed, 2 skipped, 69 subtests passed
• Ruff lint and format checks passed
• no changes under tradingagents/graph

The two skips are the optional Bedrock dependency test and an opt-in live API test without credentials.