feat(backend): add semantic matchmaking with embedding pipeline

Add a pluggable embedding provider with an offline deterministic default, vector indexing for problems and innovations, cosine semantic ranking, similar cases and solution suggestions, area filtering and an admin reindex endpoint. Refresh the index when innovations or submissions change.

backend/Application/Dtos/MatchmakingDtos.cs

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+namespace TarninaHub.Application.Dtos;
+
+public sealed record MatchRequest(
+    int? MaxResults,
+    string? ChallengeId);
+
+public sealed record SimilarCaseItem(
+    string Id,
+    string Title,
+    double Score);
+
+public sealed record SolutionItem(
+    string InnovationId,
+    string Title,
+    string Stage,
+    string ChallengeId,
+    double Score,
+    string Rationale);
+
+public sealed record MatchResponse(
+    IReadOnlyList<SimilarCaseItem> SimilarCases,
+    IReadOnlyList<SolutionItem> Solutions,
+    bool Fallback);