feat: Recently Added Guard plugin for Jellyfin 12.2

Keeps Radarr/Sonarr quality upgrades out of "Recently Added" by keeping
a ledger of the first date added per logical medium (movie or episode,
identified by provider ids and the series) and restoring it on upgrades.

- core logic without Jellyfin types (Reconciler, MediaKeys, GuardProcessor)
- thread-safe ledger with atomic, debounced persistence
- event processing via channel and background worker, seed on first start
- scheduled task "Ledger synchronisieren", status API, configuration page
- xUnit tests and a Jellyfin 12.2 integration test
- Gitea workflows: build/test, release with jprm and manifest branch

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,55 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{yml,yaml,json,xml,csproj,props,slnx}]
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false
+
+[*.cs]
+# Layout
+csharp_style_namespace_declarations = file_scoped:warning
+csharp_using_directive_placement = outside_namespace:warning
+dotnet_sort_system_directives_first = true
+csharp_new_line_before_open_brace = all
+csharp_prefer_braces = true:warning
+csharp_style_expression_bodied_methods = when_on_single_line:silent
+csharp_style_expression_bodied_properties = true:silent
+
+# var only where the type is apparent
+csharp_style_var_for_built_in_types = false:silent
+csharp_style_var_when_type_is_apparent = true:silent
+csharp_style_var_elsewhere = true:silent
+
+# Naming: private fields are _camelCase, constants PascalCase
+dotnet_naming_rule.private_fields_underscore.symbols = private_fields
+dotnet_naming_rule.private_fields_underscore.style = underscore_camel
+dotnet_naming_rule.private_fields_underscore.severity = warning
+dotnet_naming_symbols.private_fields.applicable_kinds = field
+dotnet_naming_symbols.private_fields.applicable_accessibilities = private
+dotnet_naming_style.underscore_camel.required_prefix = _
+dotnet_naming_style.underscore_camel.capitalization = camel_case
+
+dotnet_naming_rule.constants_pascal.symbols = constants
+dotnet_naming_rule.constants_pascal.style = pascal
+dotnet_naming_rule.constants_pascal.severity = warning
+dotnet_naming_symbols.constants.applicable_kinds = field
+dotnet_naming_symbols.constants.required_modifiers = const
+dotnet_naming_style.pascal.capitalization = pascal_case
+
+# Code style enforced in the build
+dotnet_diagnostic.IDE0005.severity = warning
+dotnet_diagnostic.IDE0161.severity = warning
+dotnet_diagnostic.IDE0011.severity = warning
+
+# Logging is event-driven and low-volume (like in Jellyfin itself); LoggerMessage delegates would only add noise.
+dotnet_diagnostic.CA1848.severity = none
+dotnet_diagnostic.CA1873.severity = none