feat: add indexer implementation with configuration validation

[Jenola344]

Closes #168
Closes #178

Added indexer implementation with configuration validation, Docker support, and rate-limited RPC polling

[Spagero763]

This PR's contract check failed on cspell for two reasons, both now resolved on main — no changes needed on your side: (1) your branch predates a docs fix (an unrelated word in README.md that's already allow-listed on main), and (2) the placeholder contract-ID fragments in your Docker example (CABC..., CDEF...) weren't in the project dictionary yet, so I just added them (they follow the same convention as other example IDs already there). Click "Update branch" on this PR to pick up both fixes and CI should go green.