immich: add the service and import the ZimaOS library

jupiter had a leftover docker-compose Immich on the RAID (/mnt/data/Immich,
9.9G) that survived the NixOS install. Native module now, media at
/mnt/data/AppData/immich, caddy vhost on 2283 with a 50GB body limit
(caddy's default rejects video uploads).

The package comes from nixpkgs-unstable, the module from the 26.05 pin:
26.05 ships immich 2.7.5, but that database was last written by 3.0.0 and
migrations only run forward --

  corrupted migrations: previously executed migration
  1776217577402-DropAuditTable is missing

Safe because the two module files are byte-identical at these revisions;
services/media/immich.nix carries the diff command to re-check on a bump.
Drop the input once the stable pin ships >= 3.0.0.

immich needs group "users" only to traverse /mnt/data/AppData (drwx--x---);
its own dir stays 0700 immich:immich. mediaLocation is outside /var/lib, so
the module's tmpfiles entry only ADJUSTS it -- add a rule that creates it.

scripts/immich-import-legacy-db does the database half: boots a copy of the
legacy PGDATA under the matching image (PG14 + vchord 0.3.0 + pgvector
0.8.1), dumps it with the local pg_dump 17, restores into a scratch DB,
fixes ownership, and only swaps after confirmation. Never touches the
original. The old cluster ran VectorChord, not pgvecto.rs, so the smart
search and face embeddings survive -- no ML re-run.

Imported: 666 assets, 25 people, 647 clip + 359 face embeddings, 2 users.

flake.lock

@@ -263,6 +263,22 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1784555310,
+        "narHash": "sha256-/FCliTPgiuV1owejZFNx3Ch9irdvkOfOFl+HHZ+DrtM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "421eebfd0ec7bccd4abe826ce62d7e6e83129493",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "nixpkgs_2": {
       "locked": {
         "lastModified": 1784280462,
@@ -387,6 +403,7 @@
         "mediamanager-nix": "mediamanager-nix",
         "nixos-images": "nixos-images",
         "nixpkgs": "nixpkgs_2",
+        "nixpkgs-unstable": "nixpkgs-unstable",
         "sops-nix": "sops-nix"
       }
     },