Event indexing: on-chain topics, new indexer service, optimized storage, filtering API, archiving, and query performance monitoring

[Abidoyesimze]

closes #88

Summary

Implements efficient, queryable event indexing end-to-end.
Adds missing #[ink(topic)] where helpful, introduces a Rust indexer service with Postgres storage and composite indexes, filtering REST API, Prometheus metrics, and an archiving script.
Updates docker-compose to run the full stack locally (node, Postgres, indexer).

Scope of Changes

On-chain events
contracts/staking/src/lib.rs: StakingConfigUpdated fields min_stake and reward_rate_bps are now #[ink(topic)] for off-chain filtering.
Off-chain indexer

New crate indexer/ (added to workspace):

indexer/src/ingest.rs: Subscribes to finalized blocks; records Contracts::ContractEmitted; enriches topics with emitting contract address.
indexer/src/db.rs: Postgres schema + efficient indexes; insert and filtered query functions.
indexer/src/api.rs: GET /events with filters and pagination; GET /contracts; GET /health; GET /metrics.
indexer/src/main.rs: Axum 0.7 server; Prometheus metrics; feature-gated ingestor (builds without subxt unless --features ingest).
indexer/README.md: Setup, API, metrics, archiving guidance.
Dockerfile.indexer: Multi-stage build; ingestor enabled in container via --features ingest.

Storage & archiving

Table contract_events with narrow, append-only rows and composite indexes:
Columns: block_number, block_hash, block_timestamp, contract, payload_hex, optional event_type, topics[].
Indexes: by block desc, time desc, (contract, time), (event_type, time), GIN on topics.
scripts/archive-events.sh: Moves events older than N days to events_archive.
Ops
docker-compose.yml: Adds indexer service connected to substrate-node and postgres.
Workspace updated to include indexer crate.

Acceptance Criteria Mapping

Implement event indexing for efficient querying: Done (indexer+DB+indexes).
Add event filtering capabilities: Done (/events supports contract, event_type, topic, from_ts, to_ts, from_block, to_block, limit, offset).
Optimize event storage layout: Done (narrow schema, composite/GIN indexes).
Create event archiving strategy: Done (archive-events.sh; README guidance for partitioning/Parquet).
Add event query performance monitoring: Done (/metrics via Prometheus).

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[LaGodxy]

@Abidoyesimze resolve conflicts and fix the CI.