fix(tables): make the feature-gate 404 message explicit

[andypalmi]

Summary

When FlowFuse Tables isn't enabled for a team's plan, every route under /api/v1/teams/:teamId/databases returns a plain 404: { code: 'not_found', error: 'Not Found - not available on team' }. This is indistinguishable from a genuine "that database/table doesn't exist" response.

This came up while adding FlowFuse Tables as an MCP tool for Expert (see #7990): an LLM agent calling these tools has no signal that the feature simply isn't enabled versus the resource not existing, so it can't usefully tell the user "enable Tables for your team" instead of "that doesn't exist".

We discussed two options: augmenting the response with a new reason/reasonCode field (backward compatible, but introduces a new convention with no precedent in the codebase), versus just making the existing message explicit. Went with the latter since it's a plain string with no established contract around its exact wording, matches the clearest existing precedent (forge/routes/api/assistant.js:185), and needs no new convention.

Status code and code field are unchanged, only the error message now names the feature.

Test plan

• Lint passes
• No existing test asserts the old message text

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.86%. Comparing base (5f34bbf) to head (4b1e067).

Files with missing lines	Patch %	Lines
forge/ee/routes/tables/index.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7994   +/-   ##
=======================================
  Coverage   75.86%   75.86%           
=======================================
  Files         434      434           
  Lines       23340    23340           
  Branches     6209     6209           
=======================================
  Hits        17708    17708           
  Misses       5632     5632           

Flag	Coverage Δ
backend	75.86% <0.00%> (ø)

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