Add Go package-level doc comments to src/backend/providers/

[hanneshapke]

The provider files under src/backend/providers/ all start with a bare package providers line. Go convention (and many linters such as revive/golint) want a // Package providers ... doc comment on at least one file, plus a short doc comment on each exported type/function.

Files missing package or top-of-file doc comments:

• src/backend/providers/provider.go
• src/backend/providers/openai.go
• src/backend/providers/anthropic.go
• src/backend/providers/gemini.go
• src/backend/providers/mistral.go
• src/backend/providers/custom.go

What to do

1. Add a single `// Package providers ...` comment at the top of provider.go describing what the package does (LLM provider adapters: request/response shape per provider, masking integration).
2. For each provider file, add a one-line comment above the file header (e.g. // File openai.go contains the OpenAI provider adapter.).
3. Add short // FunctionName ... doc comments to exported types and functions that currently lack them.

Hints

• Run make check or golangci-lint run before/after to confirm no new warnings.
• Keep comments short — one sentence is enough.

Difficulty: good first issue, ~1 hour.