perf: harden User and Transaction models with targeted indexes

[privexlabs]

Summary

Hardens the User and Transaction TypeORM entities for heavy-load resilience and query performance, addressing sub-optimal data fetching patterns and a type-safety gap identified in the Database & Migrations domain.

Changes

• src/models/User.model.ts: added a composite index on (userType, kycStatus) to speed up combined user-segment and verification-status lookups; corrected isKycVerified to be a required boolean (it is a NOT NULL column with a database default, but was typed as optional, allowing an incorrect undefined state at compile time).
• src/models/Transaction.model.ts: added a composite index on (status, type, timestamp) matching the exact filter/sort used by the pending Stellar-state reconciliation worker; added an index on stellar_tx_hash to speed up existing lookups performed during payment verification and reconciliation.
• src/migrations/1732300000000-AddUserAndTransactionPerformanceIndexes.ts: new migration creating the three indexes above (idempotent CREATE INDEX IF NOT EXISTS, with a matching down migration), following the repository's existing migration conventions.

Issues

Resolves #333
Resolves #334
Resolves #335
Resolves #336

Verification

• Manual code review completed: git status, git diff --stat, and the full git diff were inspected; only the two targeted model files and one new migration file are changed.
• cargo build was not run.
• cargo test was not run.
• No snapshots were committed.

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