## Spaces / Business plan — Phase 0: invisible foundation
First of **three stacked PRs** introducing multi-tenant **Spaces** (the
basis for the Business plan). This one is a **pure, behaviour-preserving
foundation**: it can ship to production on its own with zero
user-visible change.
- **Stacked PRs:** this → `enterprise-spaces-ui` (Phase 1+2) →
`enterprise-spaces-invitations` (Phase 3).
### What a Space is
A Space groups its own accounts, connections, transactions, categories,
labels, budgets and rules. **Every user gets one invisible "Personal"
space**, provisioned automatically on creation — so the architecture is
identical for free, Standard and Business accounts, even though only
Business will ever see more than one.
### What this PR does (no behaviour change)
- `spaces`, `space_user`, `space_invitations` tables;
`users.current_space_id`; a nullable, indexed `space_id` on the 8 owned
tables (plain column, **no FK** — avoids a validating table-scan/lock on
`transactions` during a phased rollout).
- `Space` model + `BelongsToSpace` trait that stamps `space_id` on
create (from the row's user's current space; a transaction inherits its
account's space, so bank-sync lands rows correctly).
- Idempotent, chunked `spaces:backfill` command (run from a migration)
that gives every existing user a personal space and stamps their rows —
so the read switch in the next PR is safe.
- **Reads are untouched here** (still user-scoped); every user has
exactly one space, so behaviour is identical.
### Testing
- New `tests/Feature/Spaces/SpaceFoundationTest.php` (provisioning,
default-space stamping, account-anchored transaction space,
stale-pointer self-heal, backfill).
- Full suite green.
### Notes / deliberate simplifications
- `space_id` stays **nullable** (populated by backfill + on every
write); the NOT NULL constraint is deferred until prod is confirmed
fully backfilled.
- For very large `transactions` tables, `spaces:backfill` can be run
out-of-band before deploy so the migration's call is a no-op.
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