fix(#45): graceful config validation on the standalone boot path

[milmazz]

What

Values the CLI's parse layer accepts for PostgREST --dump-config parity — e.g. db-max-rows=0, server-port=0 or negative (Bier's schema uses :pos_integer, i.e. >= 1) — passed --dump-config but crashed boot with a raw MatchError/ArgumentError from inside Bier.start_link/1. docker run first-contact users hit exactly this. This is the open hardening tail from the #40 review noted on #45.

How

• Bier.Config.new/2: non-raising variant of new!/2 (which now delegates to it), returning {:ok, config} | {:error, message} for both NimbleOptions schema failures and the semantic validators (jwt-secret / jwt-aud / db-channel / admin-port). Validator failures still raise ArgumentError from new!/2 as before; schema failures now raise ArgumentError too (previously NimbleOptions.ValidationError — nothing depended on that type).
• Bier.CLI.Config.validated_start_opts/1: to_start_opts/1 plus a full Bier.Config.new/2 pass, collapsing multi-line NimbleOptions union-type messages into the CLI's one-line fatal format.
• Both boot paths route through it: the escript boot in Bier.CLI.main/1 prints the message to stderr and exits 1 (same shape as the existing jwt-secret/admin-port fatals), and Bier.Application.standalone_spec/1 (the BIER_STANDALONE release/Docker path) returns the fatal message that standalone_children/1 already reports on stderr before halting.
• --dump-config parity preserved: validation runs at BOOT, not at parse/dump — the dump still prints whatever was parsed (e.g. db-max-rows = 0, server-port = -1) and exits 0, exactly as the conformance cases pin it.

Out of scope (still open on #45): --ready subcommand, db-config source, Docker smoke CI job.

Part of #45.

Verification

• mix format --check-formatted, mix compile --warnings-as-errors, MIX_ENV=test mix compile --warnings-as-errors, mix credo --strict, mix docs --warnings-as-errors, mix deps.unlock --check-unused — all clean locally.
• Exercised the new paths in-process (mix run --no-start): db-max-rows=0 / server-port=0 / server-port=-1 now yield single-line {:error, message}; valid config yields the same opts as to_start_opts/1; --dump-config with those values still exits 0.
• New unit tests in test/bier/cli/boot_validation_test.exs cover the error paths, the standalone spec, Bier.Config.new/2, and the dump parity; CI runs them (conformance suite is the judge, per repo policy).

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