fix(cat-gateway): Detect Service Outage with Health Endpoints

[saibatizoku]

Description

Defines DB connection errors, and updates middleware and endpoints to handle them by setting the relevant DB (Event DB or Index DB) liveness flags.

Closes #2349 .

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[no30bit]

lgtm

[stevenj]

Looks ok, subject to answering @no30bit questions.

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 725/729}$ | ${\color{red}Fail: 0/729}$ |

[Mr-Leshiy]

LGTM