feat: structured salary fields (min/max/currency/period)

Adds SalaryMin/SalaryMax/SalaryCurrency/SalaryPeriod alongside the
existing free-text Salary field (kept for back-compat and display).

- JobApplication model + idempotent column bridging for SQLite and MySQL
- Create/Update DTOs with NormalizeSalary (clamps negatives, swaps
  inverted min/max, uppercases currency, whitelists period)
- JobApplicationDto exposes the fields; CSV export gains 4 columns
- UI: add/edit dialogs get min/max/currency/period inputs; job table
  renders a formatted range via shared salary.ts formatter (falls back
  to free-text when structured values are absent)
- EN/NB translations; backend + full frontend suites green

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

job-tracker-ui/src/types.ts

@@ -89,6 +89,10 @@ export interface JobApplication {
   dateApplied: string;
   location?: string;
   salary?: string;
+  salaryMin?: number | null;
+  salaryMax?: number | null;
+  salaryCurrency?: string | null;
+  salaryPeriod?: string | null;
   nextAction?: string;
   followUpAt?: string;
   feedbackRequestedAt?: string;