fix(indexer): parse UserRegistered events and fix stale indexer bugs

[ykhalid218]

Description

The indexer had several stale bugs from merged branches that blocked compilation and prevented UserRegistered events from being parsed and synced to the users table.

Changes

• Fixed duplicate \YieldAccruedEvent\ struct and match arm in parser.rs (leftover from [backend] Indexer: Parse UserRegistered events and synchronize user table entries #542)
• Added missing \FriendAddedEvent\ / \FriendRemovedEvent\ structs and enum variants ([backend] Indexer: Parse UserRegistered events and synchronize user table entries #542 had the match arms but never defined them)
• Added \FriendAdded\ / \FriendRemoved\ topic filters to the Soroban RPC polling payload
• Wired up \FriendAdded\ / \FriendRemoved\ / \TokenSalvaged\ event handlers in the dispatch loop
• Fixed pre-existing import bug: \log_yield_rate_update_tx\ was called but not imported
• Fixed pre-existing test bug: \payload_subscribes_to_user_registered_events\ passed \None\ instead of &[]\
• Added \payload_subscribes_to_friend_events\ test
• Verified formatting matches rustfmt style

Closes #464

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