fix(backend): validate stellar user role and index soroban events

[Dopezapha]

Closes #310
Closes #311

PR Description

#311 fixes Stellar-authenticated user creation so newly created users always receive a valid default platform role before persistence. New Stellar users are created through a single helper that assigns UserRole.FARMER and validates that the assigned role belongs to the supported UserRole enum, preventing silent authorization issues from missing or invalid roles.

#310 adds database index support for Soroban event queries. The Soroban event query filters by contract_id, type, and ledger, then sorts by ledger and paging_token; the new migration and entity metadata add indexes for those access patterns so event lookups remain efficient as indexed event volume grows.

Changed

• #608 Ensure role is set correctly for new users in StellarStrategy #311: Centralized Stellar user creation and default role validation in StellarStrategy.
• #608 Ensure role is set correctly for new users in StellarStrategy #311: Corrected SEP-10 challenge sequence generation to produce sequence 0.
• #607 Add DB index recommendations to SorobanIndexerService #310: Added Soroban event type and composite query indexes.
• #607 Add DB index recommendations to SorobanIndexerService #310: Registered the Soroban index migration in runtime and CLI TypeORM data sources.
• #608 Ensure role is set correctly for new users in StellarStrategy #311: Updated related Stellar auth tests and DTO examples to use FARMER.

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[vic-Gray]

pls fix conflicts @Dopezapha

[Dopezapha]

Conflicts fixed