Support querying custom event properties

[chrmod]

Goal

Let the MCP query Plausible custom event properties, generically — no property names are hardcoded, so any site can slice by whatever its own tracker sends.

• Break down by a custom property: get_breakdown accepts a dimension of event:props:<name> (e.g. event:props:destination_host) alongside the standard dimensions.
• Filter by a custom property on every query tool (get_breakdown, get_timeseries, get_conversions, compare_periods) via a new property_filters input, e.g. [{ "property": "plan", "operator": "is", "values": ["pro"] }]. Operators: is, is_not, contains, contains_not; entries combine with AND.

Shared schema + filter builders live in src/schemas.ts; server instructions now document the event:props: convention to steer clients away from malformed queries.

Branched off main so it can also go upstream to getsentry/plausible-mcp later; this PR lands it on ghostery for deployment.

Test plan

In an MCP client pointed at the deployed server:

• Ask for a breakdown by a custom property your site sends (e.g. "break down visitors by destination_host") and confirm the rows are grouped by that property's values.
• Ask to filter a query by a custom property value (e.g. "daily visitors where plan is pro") and confirm the numbers are scoped to that value.
• Confirm existing queries (top pages, sources, conversions) are unchanged.