Add non-empty String and collection types

[emgrav]

Issue Description

Most inbound API payloads must reject empty strings and empty collections at the boundary. Current types allow deserialising empty strings/vectors, pushing validation deeper into business logic. We need a consistent, centralised approach so inputs fail fast (e.g. Url-like deserialisation error) and lists are guaranteed non-empty (e.g. via nonempty crate).

Options Comparison

Scope	Approach	Pros	Cons
Deserialisation layer	Custom serde validators / newtypes (NonEmptyString, TrimmedNonEmptyString) and #[serde(deserialize_with = ...)]	Fails fast at boundary; keeps invariants in types; minimal call-site changes	Requires newtype coverage of all string fields; migration effort
Collection enforcement	nonempty::NonEmpty<T> for lists in request types	Compile-time non-empty guarantee; good serde support; clear invariant in type	Type churn on existing structs; iterator ergonomics differ slightly
Mixed validation	Smart constructors post-deserialisation	Centralised logic; fewer type changes	Errors shift deeper; risks partial validation

User Story

As a client or service integrating with our APIs, I want invalid empty strings or empty lists to be rejected immediately at request parsing, so I get clear errors and the system maintains strong invariants.

Acceptance Criteria

• Provide well-documented newtypes for non-empty strings (e.g. NonEmptyString, optionally TrimmedNonEmptyString) with serde deserialisation that rejects empty/blank input at parse time.
• Provide a non-empty list type (or re-export/alias of nonempty::NonEmpty) with serde support and ergonomic constructors/iterators for library consumers.
• Ensure error messages surfaced by deserialisation are explicit and consistent (e.g. “field X must be non-empty”).
• Add unit tests for the newtypes covering empty/blank strings and empty collections, demonstrating deserialisation failures and success paths.
• Document usage in Rustdoc: how to annotate fields in consumer DTOs with these types and any helper macros/constructors.

Documentation and References

• nonempty crate docs: https://docs.rs/nonempty
• serde custom deserialisation guide: https://serde.rs/custom-date-format.html