[3] [TASK] [Backend] [Validation] Add input validation on TelemetryPingDTO

[R4R35]

Description

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Add field-level validation annotations on TelemetryPingDTO to reject structurally invalid requests before they reach the service layer. Without this, empty or malformed JSON can be saved directly to MongoDB.

Scope

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• Add @NotNull, @NotBlank on deviceId, storeId, itemId
• Add @NotNull on lat, lng, accuracyMeters, timestamp
• Add @Valid in the controller on the @RequestBody parameter
• Return a meaningful 400 Bad Request response on validation failure

Done When

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• Sending an empty JSON body returns 400 Bad Request
• Sending a request with missing fields returns 400 Bad Request
• Valid requests still return 202 Accepted and are saved correctly

Priority

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• 🔴 High