Add the media store: content-addressed blobs, SQLite index, showcase

First half of replacing the stub text domain with a media collector. Nothing
references this yet - the store is standalone and fully tested before anything
depends on it.

Blobs are addressed by SHA-256 and sharded two levels deep, so the same picture
re-uploaded at a dozen addresses costs one file. Downloads stage in a sibling
temp directory on the same volume and are promoted by rename, which is what
keeps blobs/ free of truncated files: a crash leaves a stray .part that the next
startup sweeps, never a half-image indistinguishable from a real one.

The SQLite index holds provenance separately from content, so purging one source
leaves blobs another source still references - that is what ref_count buys, and
it is recomputed rather than incremented because the item upsert can replace a
row pointing at a different blob. The seen_url journal deliberately outlives a
purge: without that, the next run downloads again exactly what the user just
deleted. Terminal outcomes are split from retryable ones so a flaky network does
not permanently lose content.

The showcase gives every item a dated, named path via hard links - a second name
for one file, not a second file. Hard links are a filesystem privilege rather
than a guarantee, so it degrades to copying and records which it achieved; the
UI has to be able to admit that. Names suggested by the origin are treated as
hostile: only the last path segment survives, Windows device names are pushed
aside, and the extension comes from the sniffed kind, never from the remote.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AvParser.Infrastructure/AvParser.Infrastructure.csproj

@@ -1,6 +1,11 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <RootNamespace>AvParser.Infrastructure</RootNamespace>
+    <!-- [LibraryImport] generates pointer-based marshalling, so the source generator needs this.
+         It is here only for the hard-link P/Invokes in Media/HardLink.cs; the BCL has no wrapper
+         for CreateHardLink/link(2). Using [DllImport] instead would trip SYSLIB1054, which is an
+         error under TreatWarningsAsErrors. -->
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <ItemGroup>
@@ -14,6 +19,7 @@
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Data.Sqlite" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" />
     <PackageReference Include="Microsoft.Extensions.Http" />
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />