fix: Post-PR #8 critical improvements (bd-64, bd-65, bd-66, bd-67)

This commit addresses all critical follow-up issues identified in the
code review of PR #8 (atomic counter implementation).

## bd-64: Fix SyncAllCounters performance bottleneck (P0)
- Replace SyncAllCounters() on every CreateIssue with lazy initialization
- Add ensureCounterInitialized() that only scans prefix-specific issues on first use
- Performance improvement: O(n) full table scan → O(1) for subsequent creates
- Add comprehensive tests in lazy_init_test.go

## bd-65: Add migration for issue_counters table (P1)
- Add migrateIssueCountersTable() similar to migrateDirtyIssuesTable()
- Checks if table is empty and syncs from existing issues on first open
- Handles both fresh databases and migrations from old databases
- Add comprehensive tests in migration_test.go (3 scenarios)

## bd-66: Make import counter sync failure fatal (P1)
- Change SyncAllCounters() failure from warning to fatal error in import
- Prevents ID collisions when counter sync fails
- Data integrity > convenience

## bd-67: Update test comments (P2)
- Update TestMultiProcessIDGeneration comments to reflect fix is in place
- Change "With the bug, we expect errors" → "After the fix, all should succeed"

All tests pass. Atomic counter implementation is now production-ready.

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

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

internal/storage/sqlite/sqlite_test.go

@@ -460,9 +460,9 @@ func TestMultiProcessIDGeneration(t *testing.T) {
 		ids[res.id] = true
 	}
 
-	// With the bug, we expect UNIQUE constraint errors
+	// After the fix (atomic counter), all operations should succeed without errors
 	if len(errors) > 0 {
-		t.Logf("Got %d errors (expected with current implementation):", len(errors))
+		t.Errorf("Expected no errors with atomic counter fix, got %d:", len(errors))
 		for _, err := range errors {
 			t.Logf("  - %v", err)
 		}
@@ -470,13 +470,9 @@ func TestMultiProcessIDGeneration(t *testing.T) {
 
 	t.Logf("Successfully created %d unique issues out of %d attempts", len(ids), numProcesses)
 
-	// After the fix, all should succeed
+	// All issues should be created successfully with unique IDs
 	if len(ids) != numProcesses {
 		t.Errorf("Expected %d unique IDs, got %d", numProcesses, len(ids))
 	}
-
-	if len(errors) > 0 {
-		t.Errorf("Expected no errors, got %d", len(errors))
-	}
 }