feat: search should also match against server description field

Problem

The ?search= parameter on /v0/servers only matches against server_name (via ILIKE substring match). It does not search the description field.

This was explicitly requested in #135 (now closed) but only the name-search portion was implemented.

Impact

AI agents and users searching for servers by what they do (e.g., "weather", "database", "GitHub integration") cannot find relevant servers unless they already know the exact server name. The description field — which contains the most useful discovery information — is completely ignored by search.

Current behavior

GET /v0/servers?search=weather

Returns only servers where server_name contains "weather". A server named io.github.example/data-tools with description "Weather data and forecasts" is not returned.

Expected behavior

Search should match against both server_name AND description fields, with name matches ranked first for relevance.

Proposed implementation

1. Add SubstringDescription filter field to ServerFilter
2. Add ILIKE condition on the description column in buildFilterConditions
3. Update the API handler to pass the search term to both filters
4. Name matches should appear before description matches (relevance ranking)

Files affected

• internal/database/database.go — add SubstringDescription to ServerFilter
• internal/database/postgres.go — add ILIKE condition for description in buildFilterConditions
• internal/api/handlers/v0/servers.go — pass search term to both name and description filters
• internal/api/handlers/v0/servers_test.go — add tests for description search
• internal/database/postgres_test.go — add tests for description filter

Context

This is particularly important for AI-agent workflows where the agent searches by capability keywords rather than knowing server names in advance.

Start with ServerFilter in internal/database/database.go and buildFilterConditions in internal/database/postgres.go, then trace how internal/api/handlers/v0/servers.go passes the search term. Add coverage in internal/api/handlers/v0/servers_test.go and internal/database/postgres_test.go. Done means /v0/servers search matches both name and description, with name matches ranked first.