fix(db): relax pending task unique index to per-(issue, agent)

The idx_one_pending_task_per_issue index only allowed one pending task
per issue across all agents, causing different agents' queued/dispatched
tasks to block each other. This mismatched the code-level dedup which
checks per (issue_id, agent_id). Replace with idx_one_pending_task_per_issue_agent
on (issue_id, agent_id) so each agent can independently have one pending task.

Fixes MUL-495

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/migrations/037_fix_pending_task_unique_index.down.sql

@@ -0,0 +1,5 @@
+DROP INDEX IF EXISTS idx_one_pending_task_per_issue_agent;
+
+CREATE UNIQUE INDEX idx_one_pending_task_per_issue
+    ON agent_task_queue (issue_id)
+    WHERE status IN ('queued', 'dispatched');

server/migrations/037_fix_pending_task_unique_index.up.sql

@@ -0,0 +1,8 @@
+-- Fix: the old index only allowed one pending task per issue across ALL agents.
+-- This caused different agents' pending tasks to block each other.
+-- Change to per-(issue, agent) so each agent can independently have one pending task.
+DROP INDEX IF EXISTS idx_one_pending_task_per_issue;
+
+CREATE UNIQUE INDEX idx_one_pending_task_per_issue_agent
+    ON agent_task_queue (issue_id, agent_id)
+    WHERE status IN ('queued', 'dispatched');