fix: cap default storage quota to available disk space (#92)

Previously the admin quota was hardcoded to 100 GB and the regular
user quota to 1 GB, regardless of actual disk capacity. On systems
with less than 100 GB free this produced misleading quota values.

Added the fs2 crate to query available disk space on the storage
filesystem. A new capped_quota() helper now returns
min(default_quota, available_disk_space) when assigning quotas
during user registration, admin setup, and OIDC provisioning.

Cargo.lock

@@ -771,6 +771,16 @@ version = "2.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "28dd6caf6059519a65843af8fe2a3ae298b14b80179855aeb4adc2c1934ee619"
 
+[[package]]
+name = "fs2"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9564fc758e15025b46aa6643b1b77d047d1a56a1aea6e01002ac0c7026876213"
+dependencies = [
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "futures"
 version = "0.3.31"
@@ -1698,6 +1708,7 @@ dependencies = [
  "chrono",
  "dotenv",
  "flate2",
+ "fs2",
  "futures",
  "hex",
  "http-body",
@@ -3292,6 +3303,28 @@ dependencies = [
  "wasite",
 ]
 
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
+
 [[package]]
 name = "windows-core"
 version = "0.62.2"