feat: encrypt private keys saved on disk and in database (#682)

Co-authored-by: Kyle Mendell <kmendell@ofkm.us>

backend/resources/migrations/postgres/20250630000000_kv_table.down.sql

@@ -0,0 +1 @@
+DROP TABLE kv;

backend/resources/migrations/postgres/20250630000000_kv_table.up.sql

@@ -0,0 +1,6 @@
+-- The "kv" tables contains miscellaneous key-value pairs
+CREATE TABLE kv
+(
+    "key"  TEXT NOT NULL PRIMARY KEY,
+    "value" TEXT
+);