feat(snapshots): add filtering by service_id and name to snapshot list endpoint

[dennisonbertram]

Problem

Snapshots Story 10 (Service Templating) and Story 14 (Cost Optimization) describe scenarios where tenants have 20+ snapshots and need to find specific ones. The list endpoint only supports limit/offset pagination, with no filtering by service_id, name, or date range.

Story 14 notes: "All accessible via GET /v1/snapshots with filtering by name" — but this filtering does not appear to be implemented.

Expected Behavior

• GET /v1/snapshots?service_id={id} — filter snapshots for a specific service
• GET /v1/snapshots?name={pattern} — filter by name (exact or prefix match)
• GET /v1/snapshots?since={unix_timestamp} — filter by creation date

Context

Discovered via UX Paths analysis (docs/ux-paths/topics/snapshots.md — Story 2, Story 10, Story 14)

[dennisonbertram]

Needs clarification: Should filtering by service_id and name be exact match or substring/prefix match? Should the filters be combinable (AND logic)? What should the query parameter names be (service_id and name, or something else)?

[dennisonbertram]

PR created: #142