fix(api): PostTemplatesTags queries database and begins transaction before team ownership check (tag enumeration oracle)

Problem

In POST /templates/tags (packages/api/internal/handlers/template_tags.go), the control plane handler initiates a database transaction via a.sqlcDB.WithTx(ctx) and executes client.GetTemplateWithBuildByTag before validating whether the authenticated user's team owns the requested template (if aliasInfo.TeamID != team.ID).

This ordering defect causes two distinct issues:

1. Tag Enumeration / Information Disclosure Oracle:
   When a user sends POST /templates/tags targeting a template owned by a different team:

   • If the requested tag does not exist on that template: GetTemplateWithBuildByTag returns sql.ErrNoRows, which triggers ErrTemplateNotFound, responding with 404 Not Found ("Template '<target>' with tag '<tag>' not found").
   • If the requested tag exists on that template: the query succeeds, and only afterwards at line 115 is aliasInfo.TeamID != team.ID checked, responding with 403 Forbidden ("You don't have access to sandbox template '%s'").
     An unauthorized tenant can therefore determine whether arbitrary private build tags (e.g., staging, v2.0.0-rc1, hotfix) exist on other teams' templates by probing the endpoint and observing the difference between HTTP 404 and HTTP 403.

2. Unnecessary Database Load and Transaction Allocation:
   Every unauthorized or invalid tag request against another tenant's template allocates a Postgres transaction connection from the pool (WithTx), acquires read locks, executes queries, and then rolls back upon failure.

Root Cause

In packages/api/internal/handlers/template_tags.go:L75-L122, tag resolution and database transaction creation precede the team ownership authorization check:

// Current flow in template_tags.go:
aliasInfo, err := a.templateCache.ResolveAlias(ctx, templateID)
if err != nil { ... }

// BUG: Transaction started and DB queried BEFORE checking aliasInfo.TeamID == team.ID
txErr := a.sqlcDB.WithTx(ctx).Exec(func(queries *queries.Queries) error {
    build, err := queries.GetTemplateWithBuildByTag(ctx, ...)
    if err != nil {
        return ErrTemplateNotFound // Returns 404 to unauthorized caller if tag missing
    }
    ...
    if aliasInfo.TeamID != team.ID {
        return a.sendAPIStoreError(c, http.StatusForbidden, ...) // Returns 403 only if tag exists
    }
})

In contrast, peer handlers such as DeleteTemplatesTags (template_tags.go:L139) and GetTemplatesTemplateIDTags (template_tags.go:L21) enforce if aliasInfo.TeamID != team.ID immediately after alias resolution:

Reproduction Steps

1. Create a template template-A under team-1 with tag v1.0.0.
2. Authenticate as an unrelated user team-2.
3. Send POST /templates/tags targeting template-A with tag: "v9.9.9" (non-existent).
   • Observed: HTTP 404 Not Found ({"code": 404, "message": "Template 'template-A' with tag 'v9.9.9' not found"})
4. Send POST /templates/tags targeting template-A with tag: "v1.0.0" (existing tag).
   • Observed: HTTP 403 Forbidden ({"code": 403, "message": "You don't have access to sandbox template 'template-A'"})
5. Expected: Both requests must return HTTP 403 Forbidden without leaking metadata about tag existence.

Technical Context

• File affected: packages/api/internal/handlers/template_tags.go
• Subsystem: Control Plane API / Templates & Tags
• Impact: Medium (Security/Privacy: cross-tenant metadata disclosure & unnecessary database transaction overhead)

Proposed Changes

Start in packages/api/internal/handlers/template_tags.go at PostTemplatesTags and compare its authorization order with GetTemplatesTemplateIDTags and DeleteTemplatesTags. Then inspect or add the focused case in packages/api/internal/handlers/template_tags_test.go and run the relevant API tests. Done means unauthorized requests consistently return 403 without querying the tag or starting a transaction.