Fix all golangci-lint issues across the codebase

[andrew]

Resolves 77 lint issues reported by golangci-lint run --enable gocritic,gocognit,gocyclo,maintidx,dupl,mnd,unparam,ireturn,goconst,errcheck. Net reduction of ~175 lines through shared helpers and deduplication.

• dupl: Extract shared ProxyUpstream, ProxyMetadata, ProxyFile on *Proxy to replace duplicate proxy methods across conda, cran, hex, go, maven, debian, rpm handlers. Extract shared test helpers for duplicate test patterns.
• errcheck: Add error discards for resp.Body.Close() and fmt.Fprint in tests.
• gocognit: Refactor 6 high-complexity functions (composer, npm, pub, pypi rewriteMetadata and MigrateSchema) by extracting helper methods.
• goconst: Define constants for repeated strings across 13 files.
• gocritic: Fix exitAfterDefer in main.go, deprecatedComment in config.go, dupBranchBody in schema.go.
• mnd: Replace ~30 magic numbers with named constants.
• unparam: Remove unused ctx parameter from updateCacheDB.