Support legacy invoice URL migration fallback (without full resolver framework)

[felixbeer]

Problem

We still need to preserve legacy invoice downloads after migration, but we do not need a full pluggable resolver framework right now.

Scope

• Keep migration simple: map legacy invoice URL data during migration.
• Support a minimal fallback path so migrated legacy invoices can still resolve invoice_url without Stripe lookup.
• Maintain current Stripe-first behavior for normal invoice entries.

Practical migration approach

• During migration, write legacy invoice URL into invoice entry data in a deterministic field.
• If stripe_invoice_id is not a Stripe ID (for example URL-like legacy value), fallback should return the stored legacy URL.

Out of scope

• General multi-provider invoice resolver architecture.

Acceptance criteria

• Migrated legacy invoices remain downloadable/viewable.
• Stripe-native invoices continue to resolve via Stripe API.
• No regression to existing invoice ZIP download behavior.