feat: Allow adding unique constraints to existing tables

[Ludv1gL]

Summary

Adding #[unique] or #[primary_key] to an existing column currently triggers AutoMigrateError::AddUniqueConstraint, forcing a full database clear to apply the schema change. This PR makes it a non-breaking migration by validating existing data first:

• If all values are unique: constraint is added seamlessly (non-breaking migration)
• If duplicates exist: migration fails with a detailed error listing up to 10 duplicate groups

Changes

• auto_migrate.rs: Replace hard AddUniqueConstraint error with CheckAddUniqueConstraintValid precheck + AddConstraint migration step
• update.rs: Implement precheck (full table scan, project constrained columns, count duplicates) and AddConstraint step execution
• relational_db.rs: Expose create_constraint() (counterpart to existing drop_constraint())
• traits.rs / datastore.rs: Add create_constraint_mut_tx to MutTxDatastore trait
• mut_tx.rs: Make create_constraint public
• formatter.rs: Format the new AddConstraint step

Safety

• Transaction safety: Precheck and constraint creation run in the same MutTx — no window for concurrent duplicate inserts
• Index creation: auto_migrate_indexes() already handles adding the backing btree index (with is_unique=true from the new schema). The constraint step only adds metadata.
• Rollback: If the precheck finds duplicates, the entire migration aborts before any changes are applied
• Error quality: Duplicate error shows table name, column names, and up to 10 example duplicate values with counts

Example error output

Precheck failed: cannot add unique constraint 'Users_email_key' on table 'Users' column(s) [email]:
3 duplicate group(s) found.
  - String("alice@example.com") appears 2 times
  - String("bob@example.com") appears 3 times
  - String("charlie@example.com") appears 2 times

Test plan

• All 12 auto_migrate tests pass
• cargo check passes for spacetimedb-schema and spacetimedb-core
• Verified the previously-expected AddUniqueConstraint error test is updated
• Manual test: add #[unique] to existing column with clean data → succeeds
• Manual test: add #[unique] to existing column with duplicates → fails with detailed error

🤖 Generated with Claude Code

[bfops]

Interesting, thank you for filing this! We'll work on getting it reviewed.