core: implement contract event replay and indexing support

[EmeditWeb]

Problem

The API cannot efficiently track historical contract
events. Soroban events are ephemeral. There is no
mechanism for the indexer to know which events it
has already processed.

What To Build

1. Add sequence numbers to all events
2. Add get_sequence() view function
3. Standardize event topic structure
4. Add get_event_count()

Files To Touch

All 5 contract events.rs, lib.rs, storage.rs files

Acceptance Criteria

• All events include sequence number
• Sequence increments on every mutation
• get_sequence() returns accurate count
• Event topics follow standard structure
• API can detect gaps in event sequence

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[Vicsygold]

BOSS PLS I NEED THIS POINT

[grantfox-oss]

🦊 GrantFox — @Vicsygold has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #17)
2. Your PR will be reviewed by the StepFi-app maintainers

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