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/Services/CaptchaService.cs

@@ -0,0 +1,58 @@
+using Microsoft.AspNetCore.DataProtection;
+
+namespace JobsMedical.Web.Services;
+
+/// <summary>
+/// A built-in, stateless math CAPTCHA (no Google reCAPTCHA — it's blocked in Iran). It renders a
+/// simple "a + b = ?" question plus a tamper-proof token (the answer + timestamp, data-protected).
+/// Verification unprotects the token, checks it hasn't expired, and compares the answer. Because
+/// the answer is encrypted in the token (not guessable/forgeable) and no server session is needed,
+/// it works across the load-balanced/containerized deploy with zero extra storage.
+/// </summary>
+public class CaptchaService
+{
+    private readonly IDataProtector _protector;
+    private static readonly TimeSpan Ttl = TimeSpan.FromMinutes(10);
+
+    public CaptchaService(IDataProtectionProvider dp) => _protector = dp.CreateProtector("hamkadr.captcha.v1");
+
+    /// <summary>Returns the question to show (e.g. "۳ + ۵") and an opaque token for a hidden field.</summary>
+    public (string Question, string Token) Create()
+    {
+        var a = Random.Shared.Next(1, 10);
+        var b = Random.Shared.Next(1, 10);
+        var token = _protector.Protect($"{a + b}|{DateTime.UtcNow.Ticks}");
+        return ($"{ToPersian(a)} + {ToPersian(b)}", token);
+    }
+
+    public bool Verify(string? token, string? answer)
+    {
+        if (string.IsNullOrWhiteSpace(token) || string.IsNullOrWhiteSpace(answer)) return false;
+        try
+        {
+            var parts = _protector.Unprotect(token).Split('|');
+            if (parts.Length != 2) return false;
+            if (long.TryParse(parts[1], out var ticks))
+            {
+                var issued = new DateTime(ticks, DateTimeKind.Utc);
+                if (DateTime.UtcNow - issued > Ttl) return false;   // expired
+            }
+            return parts[0] == ToLatin(answer).Trim();
+        }
+        catch { return false; }   // tampered / undecryptable
+    }
+
+    private static string ToPersian(int n)
+        => new string(n.ToString().Select(c => (char)('۰' + (c - '0'))).ToArray());
+
+    private static string ToLatin(string s)
+    {
+        var chars = s.ToCharArray();
+        for (var i = 0; i < chars.Length; i++)
+        {
+            if (chars[i] >= '۰' && chars[i] <= '۹') chars[i] = (char)('0' + (chars[i] - '۰'));
+            else if (chars[i] >= '٠' && chars[i] <= '٩') chars[i] = (char)('0' + (chars[i] - '٠'));
+        }
+        return new string(chars);
+    }
+}

src/JobsMedical.Web/Services/InterestService.cs

@@ -41,6 +41,10 @@ public class InterestService
     public async Task LogAsync(InterestEventType type, int shiftId)
     {
         if (string.IsNullOrEmpty(VisitorId)) return;
+        // Don't record the same applicant applying to the same shift twice (garbage prevention).
+        if (type == InterestEventType.Apply && await _db.InterestEvents.AnyAsync(e =>
+                e.VisitorId == VisitorId && e.ShiftId == shiftId && e.EventType == InterestEventType.Apply))
+            return;
         await EnsureVisitorAsync();
         _db.InterestEvents.Add(new InterestEvent
         {
@@ -54,6 +58,9 @@ public class InterestService
     public async Task LogJobAsync(InterestEventType type, int jobOpeningId)
     {
         if (string.IsNullOrEmpty(VisitorId)) return;
+        if (type == InterestEventType.Apply && await _db.InterestEvents.AnyAsync(e =>
+                e.VisitorId == VisitorId && e.JobOpeningId == jobOpeningId && e.EventType == InterestEventType.Apply))
+            return;
         await EnsureVisitorAsync();
         _db.InterestEvents.Add(new InterestEvent
         {

src/JobsMedical.Web/Services/SubmissionGuard.cs

@@ -0,0 +1,64 @@
+using System.Text.RegularExpressions;
+using JobsMedical.Web.Data;
+using JobsMedical.Web.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace JobsMedical.Web.Services;
+
+/// <summary>
+/// Anti-garbage gate for user-submitted listings: blocks duplicate positions, screens obvious
+/// spam/garbage text, and prevents the same applicant applying to a listing twice.
+/// </summary>
+public class SubmissionGuard
+{
+    private readonly AppDbContext _db;
+    public SubmissionGuard(AppDbContext db) => _db = db;
+
+    private static readonly string[] SpamMarkers =
+    {
+        "سرمایه گذاری", "سرمایه‌گذاری", "وام", "ارز دیجیتال", "رمز ارز", "فروش فالوور",
+        "بک لینک", "قرعه کشی", "کازینو", "شرط بندی", "بیت کوین", "http://", "https://", "www."
+    };
+
+    /// <summary>Spam/scam markers, mashed-keyboard, or stray links in free text (any field).</summary>
+    public static bool ContainsSpam(string? text)
+    {
+        var t = (text ?? "").Trim();
+        if (t.Length == 0) return false;
+        if (SpamMarkers.Any(m => t.Contains(m))) return true;
+        if (Regex.IsMatch(t, @"(.)\1{6,}")) return true;   // aaaaaaa / کککککککک
+        return false;
+    }
+
+    /// <summary>For jobs: a real title is required, plus the spam screen on title+description.</summary>
+    public static bool LooksLikeGarbage(string? title, string? description)
+    {
+        if (Normalize(title).Length < 3) return true;       // no real title
+        return ContainsSpam(title) || ContainsSpam(description);
+    }
+
+    /// <summary>A near-identical OPEN shift already exists at this facility?</summary>
+    public Task<bool> DuplicateShiftAsync(int facilityId, int roleId, DateOnly date,
+        TimeOnly start, ShiftType type) =>
+        _db.Shifts.AnyAsync(s => s.Status == ShiftStatus.Open && s.FacilityId == facilityId
+            && s.RoleId == roleId && s.Date == date && s.StartTime == start && s.ShiftType == type);
+
+    /// <summary>A near-identical OPEN job already exists at this facility (same role + title)?</summary>
+    public async Task<bool> DuplicateJobAsync(int facilityId, int roleId, string title)
+    {
+        var t = Normalize(title);
+        var candidates = await _db.JobOpenings
+            .Where(j => j.Status == ShiftStatus.Open && j.FacilityId == facilityId && j.RoleId == roleId)
+            .Select(j => j.Title).ToListAsync();
+        return candidates.Any(c => Normalize(c) == t);
+    }
+
+    /// <summary>Has this visitor already applied (Apply event) to this shift/job?</summary>
+    public Task<bool> AlreadyAppliedAsync(string visitorId, int? shiftId, int? jobId) =>
+        _db.InterestEvents.AnyAsync(e => e.VisitorId == visitorId
+            && e.EventType == InterestEventType.Apply
+            && ((shiftId != null && e.ShiftId == shiftId) || (jobId != null && e.JobOpeningId == jobId)));
+
+    private static string Normalize(string? s) => Regex.Replace((s ?? "").Trim(), @"\s+", " ")
+        .Replace('ي', 'ی').Replace('ك', 'ک').ToLowerInvariant();
+}