Add metadata to stations, parts and revisions

Stations, parts and revisions now carry custom metadata, the same typed key/value pairs runs and units already had, so a station's location or a part's supplier lives in TofuPilot instead of a spreadsheet next to it.

• Added a metadata editor to the station, part and revision pages: string, number and boolean values, up to 50 keys per entity, read-only for viewers.
• Added metadata to the V2 create, get and update endpoints for stations, parts and revisions. Updates are a patch: keys you don't send are kept, a null value deletes a key.
• Added a metadata filter and include_metadata to the station and part list endpoints, with per-key operators (in, contains, gte/lte/gt/lt/eq, boolean eq), so you can list every station in one building or every part from one supplier in a single call.
• Added every metadata change to the entity's timeline with who made it and the old and new value. The station Activity section now shows without a CLI install, since these edits come from the dashboard or the API.
• Shipped the new fields and list parameters in the Python, C#, Rust, C++ and MATLAB SDKs.