Add blocked_issues_cache table for GetReadyWork optimization

Introduces a materialized cache table to store blocked issue IDs,
replacing the expensive recursive CTE computation that was causing
~752ms query times on 10K databases (bd-5qim).

The cache is maintained via invalidation on dependency and status
changes, reducing GetReadyWork from O(n²) recursive traversal to
O(1) cache lookup.

Technical details:
- New blocked_issues_cache table with single issue_id column
- ON DELETE CASCADE ensures automatic cleanup
- Migration populates cache using existing recursive CTE logic
- rebuildBlockedCache() fully rebuilds cache on invalidation
- execer interface allows both *sql.DB and *sql.Tx usage

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

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

internal/storage/sqlite/migrations.go

@@ -31,6 +31,7 @@ var migrationsList = []Migration{
 	{"source_repo_column", migrations.MigrateSourceRepoColumn},
 	{"repo_mtimes_table", migrations.MigrateRepoMtimesTable},
 	{"child_counters_table", migrations.MigrateChildCountersTable},
+	{"blocked_issues_cache", migrations.MigrateBlockedIssuesCache},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -69,6 +70,7 @@ func getMigrationDescription(name string) string {
 		"source_repo_column":           "Adds source_repo column for multi-repo support",
 		"repo_mtimes_table":            "Adds repo_mtimes table for multi-repo hydration caching",
 		"child_counters_table":         "Adds child_counters table for hierarchical ID generation with ON DELETE CASCADE",
+		"blocked_issues_cache":         "Adds blocked_issues_cache table for GetReadyWork performance optimization (bd-5qim)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {