feat: Constrain transactions to one category and add tags

[nfebe]

Constrain a transaction to a single category so spend math has one unambiguous bucket, and add tags for the cross-cutting labels a second category used to stand in for. Today Transaction::categories() is a many-to-many relation (app/Models/Transaction.php:182) and there is no tag concept, so any per-category total has to either pick one of several or double count. Category becomes one-per-transaction (the spend bucket); tags become a separate user-scoped label that never feeds category, income, or expense totals. Web and mobile are blocked on this shape.

Constraint: this is the source of truth; the web and mobile follow-ups depend on the model and endpoints defined here.

Checklist

• Move a transaction to exactly one category (single association), keeping category type (income/expense) intact.
• Add a tag model and endpoints: user-scoped, many-to-many on transactions, independent of categories.
• /stats builds the category breakdown from the single category; tags never contribute to any total.
• Migrate existing multi-category transactions: keep one category, move the rest to tags.
• Transaction serialization and sync expose the one category and the tag list.

Acceptance criteria

• A transaction has at most one category and any number of tags.
• /stats category totals sum to total expense (and income) with no double counting.