Issue #67 — Share position repository

[GoSTEAN]

Title: [Repository] Implement share position repository

Tags: backend, repository, trading, feature

Description:
Dedicated repository for reading and writing user share positions, used by trading and redemption flows.

Acceptance Criteria:

• share.repository.ts provides: getPosition(userId, marketId, outcomeId), upsertPosition, getAllForUser(userId), getAllForMarket(marketId), deletePosition.
• upsertPosition is atomic — uses DB ON CONFLICT DO UPDATE.
• getAllForUser supports pagination and status filter (open, redeemed).
• All queries select only needed columns (avoid SELECT *).
• Unit test: upsert new → upsert again → single record updated correctly.

File: src/repositories/share.repository.ts

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