Standardize JSON tags to camelCase across all structs

Convert all existing snake_case JSON tags to camelCase and add camelCase tags to previously untagged public API types. This establishes a consistent JSON convention across the codebase following standard Go ecosystem practices.

Output types (Result, ProbeResult, FetchResult, BranchPlan, etc.) and input types (Endpoint, RefMapping, SyncPolicy, etc.) now all use camelCase. Custom MarshalJSON methods updated to match.

Auth/OAuth response structs in internal/auth/entiredb.go are intentionally left unchanged as they match external API formats.