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/__tests__/db-utils.test.ts

@@ -1,11 +1,18 @@
 import { describe, it, expect } from "vitest";
 import { isTransientError } from "../utils/errors.js";
 
+/** Create an Error with a `.code` property (like Node/pg errors) */
+function makeError(opts: { code?: string; message?: string }): Error {
+  const err = new Error(opts.message || "");
+  if (opts.code) (err as Error & { code: string }).code = opts.code;
+  return err;
+}
+
 describe("isTransientError", () => {
   describe("transient error codes", () => {
     for (const code of ["ECONNRESET", "ECONNREFUSED", "EPIPE", "ETIMEDOUT", "CONNECTION_LOST", "57P01", "57P02", "57P03", "08006", "08003", "08001"]) {
       it(`detects code ${code}`, () => {
-        expect(isTransientError({ code })).toBe(true);
+        expect(isTransientError(makeError({ code }))).toBe(true);
       });
     }
   });
@@ -20,7 +27,7 @@ describe("isTransientError", () => {
 
   describe("non-transient errors", () => {
     it("rejects generic error", () => expect(isTransientError(new Error("something broke"))).toBe(false));
-    it("rejects SQL syntax error", () => expect(isTransientError({ code: "42601", message: "syntax error" })).toBe(false));
+    it("rejects SQL syntax error", () => expect(isTransientError(makeError({ code: "42601", message: "syntax error" }))).toBe(false));
   });
 
   describe("null/undefined input", () => {
@@ -29,8 +36,9 @@ describe("isTransientError", () => {
   });
 
   describe("partial error objects", () => {
-    it("handles error with code but no message", () => expect(isTransientError({ code: "ECONNRESET" })).toBe(true));
-    it("handles error with message but no code", () => expect(isTransientError({ message: "connection terminated" })).toBe(true));
-    it("rejects error with unrelated code and no message", () => expect(isTransientError({ code: "UNKNOWN" })).toBe(false));
+    it("handles Error with code but no message", () => expect(isTransientError(makeError({ code: "ECONNRESET" }))).toBe(true));
+    it("handles Error with message but no code", () => expect(isTransientError(new Error("connection terminated"))).toBe(true));
+    it("rejects Error with unrelated code and no message", () => expect(isTransientError(makeError({ code: "UNKNOWN" }))).toBe(false));
+    it("rejects plain object (not an Error instance)", () => expect(isTransientError({ code: "ECONNRESET" })).toBe(false));
   });
 });