Bugfixes. Webclient. DataProvider ...

Server:
- Models/Devices: alias_is_uniq excludes removed devices
- ErrorHandler: skip error page output if headers already sent

Webclient:
- DataProvider: getOrFetch, invalidate, invalidatePrefix, getCollection
- Areas cache: AreasApi.list caches raw.areas.list; placeInArea uses getOrFetch
- Cache invalidation after area/device mutations
- areaDetailsModal reads from DataProvider instead of DOMObject.provideData
- areas-tree: fix self-referencing and cycle-breaking in makeTree/renderingBranch
- placeInArea: fix unassign button not appearing after first place action
- index.php: safe TEXTMSGS parsing via PHP json_decode, no JS template literal injection
- webclient/config.php: version bump, server URL updated
- DOCS.md: DataProvider full API reference