fix: preserve project route model inheritance

[numnx]

What changed

• Preserve inherited route provider fields when a project or sprint stores a sparse provider override.
• Keep scoped provider-pool replacement semantics while merging retained provider entries field by field.
• Add backend and dashboard regressions for changing only a project route thinking mode.
• Document route provider inheritance across system, project, and sprint scopes.

Why

Project settings are stored as sparse overrides. Route provider maps were replacing inherited entries atomically during resolution, so a project-only thinking override discarded the system route model. The route then fell back to the provider instance's base model, and unsupported thinking values could also be sanitized away.

With this change, provider IDs still define the narrower scope's pool, but each retained provider entry inherits its model, thinking mode, enabled state, and weight independently. Existing sparse overrides recover automatically without a database migration or re-save.

Validation

• pnpm run ci
• git diff --check
• Runtime /health and /ready checks
• Live effective-settings API verification for an existing sparse project override

Risk and rollback

Risk is limited to scoped route-resolution behavior. Existing tests retain the intentional provider-pool replacement semantics. Rollback is the single commit in this PR; no schema or data migration is involved.