[Backend] MongoDB integration + OOP repository pattern in Symfony service

[sdntsng]

objective

implement mongodb-backed persistence for the symfony adapter using explicit repository interfaces and domain entities/value objects where appropriate.

why

• align with target roles requiring mongodb + oop/design patterns
• keep data access concerns isolated/testable

scope

• mongodb client wiring and config
• repository interfaces + concrete implementations
• query pagination/filtering for contacts/leads
• basic indexing strategy doc

acceptance criteria

• create/read/update flows persist in mongodb
• repository abstraction covered by tests
• docs include setup and schema assumptions

dependency

blocked by: #91

resume evidence

can be cited as mongodb + oop repository implementation evidence.