FEAT-NAMEGEN: GET /names/suggest backend (Gemini, 503 NamesKeyMissing, LitterLetter)

- NameSuggestionService: Gemini via existing OpenAiChatClient, builds
  letter/gender/usages/count prompt, strips markdown fences from response,
  deserialises [{name,meaning,origin}] array; NotConfigured -> 503.
- GET /names/suggest?letter=&gender=&usages=&count= -> Ok<List<NameSuggestion>>
  | 503 {code:NamesKeyMissing} | 502 {code:NamesUpstreamError}.
- Litter.LitterLetter (string?, nullable) + AddLitterLetter migration.
- 17 new tests (prompt assembly, fence strip, parse edge cases,
  503-not-configured, upstream-error); total 157/157 green.
- No Behind-the-Name dependency — Gemini path only (Julian's decision).

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

GerbilManagerWebAPI/Names/NameSuggestion.cs

@@ -0,0 +1,5 @@
+namespace GerbilManagerWebAPI.Names
+{
+    /// <summary>FEAT-NAMEGEN: a single name suggestion returned by GET /names/suggest.</summary>
+    public sealed record NameSuggestion(string Name, string Meaning, string Origin);
+}

GerbilManagerWebAPI/Names/NameSuggestionService.cs

@@ -0,0 +1,107 @@
+using System.Text;
+using System.Text.Json;
+using GerbilManagerWebAPI.Ai;
+using GerbilManagerWebAPI.SaleAd;
+using Microsoft.Extensions.Options;
+
+namespace GerbilManagerWebAPI.Names
+{
+    /// <summary>
+    /// FEAT-NAMEGEN: generates meaningful gerbil name suggestions via Gemini
+    /// (the same OpenAiChatClient used by sale-ads and reply-drafts).
+    /// Returns NotConfigured when the AI section is missing — callers map to 503.
+    /// </summary>
+    public sealed class NameSuggestionService(HttpClient http, IOptions<AiOptions> options)
+    {
+        private readonly OpenAiChatClient _client = new(http, options);
+
+        private static readonly JsonSerializerOptions JsonOpts = new()
+        {
+            PropertyNameCaseInsensitive = true,
+        };
+
+        public async Task<NameSuggestionResult> SuggestAsync(
+            string? letter, string? gender, string? usages, int count,
+            CancellationToken ct = default)
+        {
+            var aiResult = await _client.CompleteAsync(
+                BuildSystemPrompt(),
+                BuildUserPrompt(letter, gender, usages, count),
+                ct);
+
+            if (aiResult.Status == AiCallStatus.NotConfigured)
+                return new NameSuggestionResult(NameSuggestionStatus.NotConfigured, null, aiResult.Error);
+            if (aiResult.Status != AiCallStatus.Ok || aiResult.Text is null)
+                return new NameSuggestionResult(NameSuggestionStatus.UpstreamError, null, aiResult.Error);
+
+            var suggestions = ParseSuggestions(aiResult.Text);
+            return suggestions is null
+                ? new NameSuggestionResult(NameSuggestionStatus.UpstreamError, null, "Ungültiges JSON in KI-Antwort.")
+                : new NameSuggestionResult(NameSuggestionStatus.Ok, suggestions, null);
+        }
+
+        internal static string BuildSystemPrompt() =>
+            "Du bist ein Helfer für Rennmaus-Züchter. " +
+            "Antworte IMMER mit einem reinen JSON-Array — KEINE Markdown-Code-Blöcke, " +
+            "KEINE Erklärungen, KEIN Text außerhalb des Arrays. " +
+            "Jedes Element hat genau die Felder: name, meaning, origin (alle Strings, alle auf Deutsch).";
+
+        internal static string BuildUserPrompt(string? letter, string? gender, string? usages, int count)
+        {
+            var sb = new StringBuilder();
+            sb.Append($"Schlage {count} Rennmaus-Namen vor");
+            if (!string.IsNullOrWhiteSpace(letter))
+                sb.Append($" die mit dem Buchstaben \"{letter.ToUpperInvariant()}\" beginnen");
+            if (!string.IsNullOrWhiteSpace(gender) &&
+                !gender.Equals("any", StringComparison.OrdinalIgnoreCase))
+                sb.Append($", passend für {(gender.Equals("female", StringComparison.OrdinalIgnoreCase) ? "weibliche" : "männliche")} Tiere");
+            if (!string.IsNullOrWhiteSpace(usages))
+                sb.Append($", aus den Kulturkreisen: {usages}");
+            sb.Append(". Jeder Name muss eine echte etymologische Bedeutung und Herkunft haben ");
+            sb.Append("(keine erfundenen oder zufälligen Namen). ");
+            sb.Append($"Antworte mit genau {count} Elementen als reines JSON-Array: ");
+            sb.Append("[{\"name\":\"...\",\"meaning\":\"...\",\"origin\":\"...\"}]");
+            return sb.ToString();
+        }
+
+        /// <summary>
+        /// Strips optional markdown fences (```json ... ```) Gemini sometimes wraps around
+        /// its JSON output, then deserialises the array.
+        /// </summary>
+        internal static List<NameSuggestion>? ParseSuggestions(string raw)
+        {
+            var text = raw.Trim();
+
+            // Strip ```json ... ``` or ``` ... ``` fences.
+            if (text.StartsWith("```", StringComparison.Ordinal))
+            {
+                var firstNewline = text.IndexOf('\n');
+                if (firstNewline >= 0) text = text[(firstNewline + 1)..];
+                if (text.EndsWith("```", StringComparison.Ordinal))
+                    text = text[..^3].TrimEnd();
+            }
+
+            // Find the JSON array bounds defensively.
+            var start = text.IndexOf('[');
+            var end = text.LastIndexOf(']');
+            if (start < 0 || end <= start) return null;
+            text = text[start..(end + 1)];
+
+            try
+            {
+                return JsonSerializer.Deserialize<List<NameSuggestion>>(text, JsonOpts);
+            }
+            catch (JsonException)
+            {
+                return null;
+            }
+        }
+    }
+
+    public enum NameSuggestionStatus { Ok, NotConfigured, UpstreamError }
+
+    public sealed record NameSuggestionResult(
+        NameSuggestionStatus Status,
+        List<NameSuggestion>? Suggestions,
+        string? Error);
+}