refactor: eliminate all catch(err: any) with proper unknown typing + type email transport

- Replace all catch(err: any) with catch(err: unknown) across 8 source files
- Add errorMessage() and errorCode() helpers for safe error property access
- Type nodemailer transport config as SMTPTransport.Options (was any)
- Type health endpoint databaseStatus (was any)
- Type convert route margin param (was any)
- Change queryWithRetry params from any[] to unknown[]
- Update isTransientError to require Error instances (was accepting plain objects)
- 19 new TDD tests (error-type-safety.test.ts)
- Updated existing tests to use proper Error instances
- 598 tests total, all passing, zero type errors

src/utils/errors.ts

@@ -16,10 +16,10 @@ const TRANSIENT_ERRORS = new Set([
 /**
  * Determine if an error is transient (PgBouncer failover, network blip)
  */
-export function isTransientError(err: any): boolean {
-  if (!err) return false;
-  const code = err.code || "";
-  const msg = (err.message || "").toLowerCase();
+export function isTransientError(err: unknown): boolean {
+  if (!(err instanceof Error)) return false;
+  const code = (err as Error & { code?: string }).code || "";
+  const msg = err.message.toLowerCase();
   
   if (TRANSIENT_ERRORS.has(code)) return true;
   if (msg.includes("no available server")) return true;
@@ -30,3 +30,20 @@ export function isTransientError(err: any): boolean {
   
   return false;
 }
+
+/**
+ * Extract message from unknown error (safe for catch blocks).
+ */
+export function errorMessage(err: unknown): string {
+  if (err instanceof Error) return err.message;
+  if (typeof err === "string") return err;
+  return String(err);
+}
+
+/**
+ * Extract error code from unknown error (e.g., PostgreSQL/Node error codes).
+ */
+export function errorCode(err: unknown): string | undefined {
+  if (err instanceof Error && "code" in err) return (err as Error & { code?: string }).code;
+  return undefined;
+}