Add number of indexed indicators to available datasets tool output

[Fedir-Yatsenko]

Track and display the number of indicator sub-series:

• Across all datasets (total count shown at the top of "available datasets" tool output)
• Per specific dataset (added to each dataset entry in the tool output)
• This feature should be enabled via a dedicated flag in the configuration

Acceptance criteria

• "What data do you have?" query returns "X indicators available" (total across all datasets)
• Per-dataset indicator count is included in the "available datasets" tool response
• Feature is controlled by a dedicated configuration flag
• Channels without the flag enabled still work correctly (no indicator counts shown)

Proposed approach

• Calculate indicator count per dataset by querying the count of rows in the indicator metadata table, filtered by the dataset's version ID
• Aggregate per-dataset counts to get the total indicator count across all datasets
• Add total indicator count to the top of "available datasets" tool output
• Add per-dataset indicator count to each dataset entry in "available datasets" tool output