Add anti-abuse: built-in captcha + garbage/duplicate guard

- CaptchaService: stateless data-protected math captcha (no Google reCAPTCHA — blocked in Iran), TTL + Persian-digit tolerant; on PostJob + PostShift
- SubmissionGuard: duplicate-position detection (facility+role+date/time for shifts, facility+role+title for jobs), spam/garbage screen on title/description, double-apply prevention
- InterestService: Apply events deduped so an applicant can't apply to the same listing twice
- Verified: wrong captcha rejected, correct publishes, duplicate + garbage blocked

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/JobsMedical.Web/Pages/Employer/PostJob.cshtml

@@ -77,6 +77,11 @@
                 <label>شرایط احراز</label>
                 <textarea name="Requirements" rows="2">@Model.Requirements</textarea>
             </div>
+            <div class="filter-group">
+                <label>سؤال امنیتی: حاصل <strong>@Model.CaptchaQuestion</strong> چند می‌شود؟</label>
+                <input type="text" name="CaptchaAnswer" dir="ltr" inputmode="numeric" autocomplete="off" placeholder="پاسخ" />
+                <input type="hidden" name="CaptchaToken" value="@Model.CaptchaToken" />
+            </div>
             <button type="submit" class="btn btn-accent btn-block btn-lg">انتشار موقعیت</button>
         </form>
     }