fix(ci): replace deprecated chi RealIP + rename gomodguard linter

[aanogueira]

Summary

Two Go-side fixes that were authored as part of #28 but didn't make it into the merge (the PR was merged before these two later commits were pushed). Same content; re-opening cleanly off main.

1. `chi` middleware.RealIP deprecated

Deprecated for spoofing the X-Forwarded-For header even when upstreams don't strip it (GHSA-3fxj-6jh8-hvhx, GHSA-rjr7-jggh-pgcp, GHSA-9g5q-2w5x-hmxf), failing the `go-checks` lint job.

Replace with `internal/middleware.RealIP`, which only honors XFF when the immediate TCP peer is in a configured trusted-proxy CIDR set:

• New `server.trusted_proxies` config field (list of CIDRs / bare IPs)
• Empty config = no-op middleware; r.RemoteAddr keeps the actual TCP peer (safe default)
• Indexer's internal HTTP server doesn't need client-IP rewriting → line removed there entirely
• 12 unit tests covering: untrusted peer, trusted peer with single XFF, multi-hop chain, all-trusted edge case, IPv6 parsing
• `mustParseTrustedProxies` panics on invalid CIDR at startup so misconfiguration fails fast

2. `gomodguard` linter renamed to `gomodguard_v2`

golangci-lint v2.12.0 deprecation. Explicit enable + disable for the old name silences the warning.

Verification (locally on this branch)

• `go build ./...` ✓
• `go test ./internal/middleware/... ./internal/config/...` — 33 pass ✓
• `golangci-lint run ./...` — no issues ✓

Test plan

• `go-checks` passes (lint + tests)
• All `docker-build` jobs still pass (no Dockerfile change here)

Follow-up

• Once deployed behind ingress, set `server.trusted_proxies` to the ingress/LB CIDR in production config so request logs show the real client IP again