fix(backend): apply auto-tag rule to existing files on creation

CreateRule accepted apply_to_existing but ignored it, so enabling the
checkbox while creating a rule never retroactively tagged files already
carrying the when-tag — only activating an existing rule did. Extract the
retroactive expansion into TagRuleRepo.ApplyToExisting (reused by SetActive)
and call it from CreateRule when the rule is active, inside one transaction
so a file is never left half-tagged. Mirrors SetRuleActive semantics,
including following only active downstream rules.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/port/repository.go

@@ -101,6 +101,10 @@ type TagRuleRepo interface {
 	// are both true, the full transitive expansion of thenTagID is retroactively
 	// applied to all files that already carry whenTagID.
 	SetActive(ctx context.Context, whenTagID, thenTagID uuid.UUID, active, applyToExisting bool) error
+	// ApplyToExisting retroactively applies the full transitive expansion of
+	// thenTagID (following active rules) to every file that already carries
+	// whenTagID. Used when a rule is created or activated with apply_to_existing.
+	ApplyToExisting(ctx context.Context, whenTagID, thenTagID uuid.UUID) error
 	Delete(ctx context.Context, whenTagID, thenTagID uuid.UUID) error
 }