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/Program.cs

@@ -18,6 +18,8 @@ builder.Services.AddScoped<VisitorContext>();
 builder.Services.AddScoped<InterestService>();
 builder.Services.AddScoped<RecommendationService>();
 builder.Services.AddScoped<OtpService>();
+builder.Services.AddSingleton<CaptchaService>();
+builder.Services.AddScoped<SubmissionGuard>();
 // Listing parser: heuristic now; swap for an LLM-backed IListingParser later.
 builder.Services.AddSingleton<IListingParser, HeuristicListingParser>();