fix: scope Stellar projected events by network

[edehvictor]

Closes #201

Changes

• adds immutable network provenance to projected Stellar events
• keys projected events as network plus Horizon operation ID to avoid testnet/mainnet collisions
• filters live Stellar activity queries, including admin queries, by the configured network and active projection status
• adds a migration script to backfill legacy projected rows from raw network provenance or quarantine ambiguous records

Testing

• Not run per requested immediate push.