Frontend for the pluggable entity metrics endpoint (Inputs page)

[ousmaneo]

Wires the new GET /system/inputs/metrics endpoint into the Inputs overview as three opt-in, hidden-by-default columns. No metrics request fires unless the user enables one of them via the column picker.

New columns

Column	Source	Expanded section
Message Count (Last 24H)	messages_per_stream (sum of values)	per-stream breakdown — stream titles resolved via POST /system/catalog/entities/titles, deleted streams shown as <id> (deleted)
Extractors	extractor_count	lazy-loaded list from GET /system/inputs/{inputId}/extractors; "Manage extractors" lives in the section's actions slot
Associated Streams (Last 24H)	derived from messages_per_stream keys	linked list of stream titles

Each cell renders as a CountBadge with a chevron that flips when the matching section is open. While metrics are loading the cell shows a spinner; on error or missing data it falls back to — — the table itself never blocks on the metrics fetch (two-phase render).

Scope

Inputs page only. Streams, Forwarder Inputs, and Event Definitions columns from the broader project PRD are out of scope for this PR and will land with their own backend endpoints.
/nocl
associated to #25997
part of #25928

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.