Add PairingService with safe alphabet and 10-min TTL

backend.tests/PairingServiceTests.cs

@@ -0,0 +1,28 @@
+using backend.Pairing;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class PairingServiceTests
+{
+    [Fact]
+    public void Generated_codes_use_only_safe_alphabet_and_are_8_chars()
+    {
+        var svc = new PairingService();
+        for (var i = 0; i < 50; i++)
+        {
+            var code = svc.GenerateCode();
+            code.Length.Should().Be(8);
+            code.Should().MatchRegex("^[A-HJ-KM-NP-Z2-9]+$");
+        }
+    }
+
+    [Fact]
+    public void Generated_codes_are_not_all_identical()
+    {
+        var svc = new PairingService();
+        var codes = Enumerable.Range(0, 20).Select(_ => svc.GenerateCode()).ToHashSet();
+        codes.Count.Should().BeGreaterThan(15);
+    }
+}

backend/Pairing/PairingService.cs

@@ -0,0 +1,84 @@
+using System.Security.Cryptography;
+using backend.Data;
+using backend.Devices;
+using Microsoft.EntityFrameworkCore;
+
+namespace backend.Pairing;
+
+public record PairResult(Guid DeviceId, string DeviceToken);
+
+public class PairingService
+{
+    private const string Alphabet = "ABCDEFGHJKMNPQRSTUVWXYZ23456789"; // no O, 0, I, 1, L
+    private const int CodeLength = 8;
+    public static readonly TimeSpan CodeTtl = TimeSpan.FromMinutes(10);
+
+    private readonly AppDbContext? _db;
+    private readonly DeviceTokenService? _tokens;
+
+    public PairingService() { }
+
+    public PairingService(AppDbContext db, DeviceTokenService tokens)
+    {
+        _db = db;
+        _tokens = tokens;
+    }
+
+    public string GenerateCode()
+    {
+        Span<byte> bytes = stackalloc byte[CodeLength];
+        RandomNumberGenerator.Fill(bytes);
+        Span<char> chars = stackalloc char[CodeLength];
+        for (var i = 0; i < CodeLength; i++)
+            chars[i] = Alphabet[bytes[i] % Alphabet.Length];
+        return new string(chars);
+    }
+
+    public async Task<PairingCode> IssueAsync(Guid ownerUserId, string deviceName, CancellationToken ct)
+    {
+        if (_db is null) throw new InvalidOperationException("PairingService not constructed with DB context.");
+        var entity = new PairingCode
+        {
+            Code = GenerateCode(),
+            OwnerUserId = ownerUserId,
+            DeviceName = deviceName,
+            ExpiresAt = DateTimeOffset.UtcNow.Add(CodeTtl),
+        };
+        _db.PairingCodes.Add(entity);
+        await _db.SaveChangesAsync(ct);
+        return entity;
+    }
+
+    public async Task<PairResult?> ConsumeAsync(string code, CancellationToken ct)
+    {
+        if (_db is null || _tokens is null)
+            throw new InvalidOperationException("PairingService not constructed with DB context + token service.");
+        var now = DateTimeOffset.UtcNow;
+        var pc = await _db.PairingCodes.FirstOrDefaultAsync(p => p.Code == code, ct);
+        if (pc is null || pc.ConsumedAt is not null || pc.ExpiresAt < now)
+            return null;
+
+        var (plaintext, hash) = _tokens.Generate();
+        var defaults = await _db.SystemSettings.FirstAsync(ct);
+
+        var device = new Device
+        {
+            OwnerUserId = pc.OwnerUserId,
+            Name = pc.DeviceName,
+            PairedAt = now,
+            TokenHash = hash,
+        };
+        device.Config = new DeviceConfig
+        {
+            DeviceId = device.Id,
+            SystemPrompt = defaults.DefaultSystemPrompt,
+            Voice = defaults.DefaultVoice,
+            Model = defaults.DefaultModel,
+            IdleTimeoutSeconds = defaults.DefaultIdleTimeoutSeconds,
+        };
+        _db.Devices.Add(device);
+        pc.ConsumedAt = now;
+        await _db.SaveChangesAsync(ct);
+        return new PairResult(device.Id, plaintext);
+    }
+}