ci(preview): harden the branch-reset scripts (PR #255 follow-ups)

[JeremyFunk]

The four non-blocking follow-ups from the #255 review:

1. Tripwire on normalize-preview-ownership.ts — same CI || RESET_PREVIEW_CONFIRM=1 guard as the reset script. Ownership reassignment is low-harm, but bun auto-loads .env and mise loads .env.local, so a dev shell can silently carry a real DATABASE_URL; the script now refuses outside CI without explicit confirmation.
2. Wider emptiness verification in reset-preview-branch.ts — the post-drop check now counts routines and standalone enum/domain types alongside pg_class relations. Previously a surviving enum (e.g. an owner the reset couldn't assume) passed verification and only failed later as duplicate_object at migrate replay — past the point where the delete → recreate fallback exists.
3. RESET_PRESERVE_SCHEMAS escape hatch — comma-separated schema names exempted from the extra-schema drop, so a future postgres-owned PlanetScale vendor schema can be protected without a code change.
4. Dead-code cleanup in electric-pr-branch.ts — sourceId resolves via ?? fail(...) so it's typed string outright; the dead if (sourceId) wrapper and downstream re-checks are gone.

Verification: the four scripts pass tsc --strict (they're excluded from CI typecheck as bun runtime scripts), and both tripwires + the usage guard were smoke-run locally (refuse with exit 1 before any connection is made).

🤖 Generated with Claude Code

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