Migrate byte-only path contracts to `pathlib.Path`

[snejus]

Parent issue: #1409

Depends on: the generic path utilities and initial test-suite migration sub-issues.

Problem

Core runtime interfaces still expose filesystem paths as bytes, propagating encoding and byte-valued model fields through import, artwork, plugin, and UI code. The remaining tests therefore cannot adopt Path.

Affected areas

• Library/database integration: Library.directory, music-directory context, path queries, and byte-valued model backing fields.
• Import pipeline: ImportSession, task classes and factories, and persisted ImportState.
• Artwork/media utilities: fetchart.Candidate.path, ArtResizer, extension handling, and shared art helpers.
• Plugins/UI: direct model-field access, importer paths, and path configuration values.

Expected behavior

• Use Path for filesystem paths in runtime and plugin APIs.
• Prefer LibModel.filepath and Album.art_filepath; access their byte-valued backing fields only at the database compatibility boundary.
• Migrate importer state and APIs, artwork utilities, plugin state, and event payloads to Path, while decoding legacy state.
• Encode or decode only at explicit boundaries. SQLite path BLOBs and binary file or URI content remain bytes.
• Preserve undecodable filesystem paths through surrogate-escape round trips.

Acceptance criteria

• Direct LibModel.path and Album.artpath use is limited to persistence and compatibility code.
• Filesystem, importer, and artwork APIs accept or return Path without caller-side encoding.
• Art, importer, core, and plugin tests migrate with their production interfaces.
• Boundary tests cover database round trips, legacy state, undecodable paths, and platform behavior.

It's just a high-level issue - it should be split into more specific sub-issues.