feat(ai): add an opper provider alias

[Felixkw12]

What and why

Adds Opper (an EU-hosted, OpenAI compatible LLM gateway) as a PGBOT_AI_PROVIDER alias for explain and ask, wired the same way as the existing requesty and openrouter aliases. No new client code: it reuses the /chat/completions provider and only picks the endpoint and key variable.

Changes:

• internal/ai/openai.go: defaultOpperURL = "https://api.opper.ai/v3/compat" next to defaultRequestyURL.
• internal/ai/resolve.go: opper joins the /chat/completions case, maps to its endpoint, and reads its key from OPPER_API_KEY (or PGBOT_AI_API_KEY) only, so an OpenAI or OpenRouter key in the shell is never sent to it. The requesty if/else became a small switch for that. keyVarsFor names OPPER_API_KEY in the missing key error. It is not added to auto detection.
• internal/ai/resolve_test.go: TestResolve_opper, the requesty test with the names swapped (both key variables, refusal to borrow OPENAI_API_KEY, no auto detection).
• README environment table and explain section, plus a changelog entry under Unreleased.

The default gpt-5.6-terra is served by Opper under the same id, with reasoning_effort up to max, so PGBOT_AI_MODEL is optional.

How to test:

export PGBOT_AI_PROVIDER=opper
export OPPER_API_KEY=...            # https://platform.opper.ai
export PGBOT_AI_MODEL=claude-sonnet-4-6   # optional
pgbot ask "What needs attention?" "$DATABASE_URL"

Validation:

• I don't have a Go toolchain on this machine, so scripts/gate.sh, go test and gofmt -l were not run here. The change copies the requesty alias and its test line for line.

Disclosure: Opper is my startup.

Checklist

• scripts/gate.sh passes (builds HEAD, not just the working tree) (not run locally, see above)
• New SQL is read-only; no EXPLAIN ANALYZE; findings stay deterministic (computed in Go) (no SQL touched)
• No PII enters a model.Context / --json / the store (unchanged)
• --json change is additive, or model.SchemaVersion bumped + schema regenerated (go run ./tools/schemagen) (no --json change)
• A new finding has a docs/findings/<id>.md page + catalog entry (no new finding)