docs: align platform documentation and add clarifying comments

[enekoreto]

Description

This PR standardises and corrects documentation for the current platform behaviour, and adds concise clarifying comments/docstrings in core files where logic was non-obvious.

Scope

• Updated top-level project documentation and added an E2E runbook.
• Corrected Terraform documentation to reflect implemented routes/modules (including history_list).
• Rewrote bootstrap documentation to match the actual bootstrap folder and backend values.
• Added lightweight fixture documentation.
• Added brief explanatory comments/docstrings in workflow, frontend utilities, worker, Lambda handlers, and API client scripts.

Behavioural impact

• No functional or infrastructure behaviour changes intended.
• Changes are documentation/commentary only.

Validation

• Verified markdown links resolve across tracked documentation files.
• Confirmed branch CI status: Terraform CI passed.

Reviewer notes

Please review for clarity, accuracy, and consistency with current team operating policy (shared dev infrastructure preserved; EC2-only E2E cleanup).

Hi, the requested changes have been implemented, so I am dismissing the remarks now.

[prathikanand7]

Closing this PR.