🔐 feat: Add token export/import for ephemeral environments

- Add token CLI subcommand with export/import operations
- Enable OAuth2 token persistence across clean environments
- Support for containers, CI/CD, and ephemeral compute workflows
- Compress tokens with gzip and encode as base64 for env vars
- Automatic token restoration from CULL_GMAIL_TOKEN_CACHE
- Secure file permissions (600) on restored tokens
- Add comprehensive error handling for token operations
- Update dependencies: base64, flate2, serde_json

This feature enables cull-gmail to run in ephemeral environments
like Docker containers and CI/CD pipelines without re-authentication
by exporting tokens once and restoring them via environment variables.

Cargo.lock

@@ -352,6 +352,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "crc32fast"
+version = "1.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9481c1c90cbf2ac953f07c8d4a58aa3945c425b7185c9154d67a65e4230da511"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "crossbeam-utils"
 version = "0.8.21"
@@ -372,11 +381,13 @@ dependencies = [
 name = "cull-gmail"
 version = "0.0.11"
 dependencies = [
+ "base64",
  "chrono",
  "clap",
  "clap-verbosity-flag",
  "config",
  "env_logger",
+ "flate2",
  "futures",
  "google-gmail1",
  "httpmock",
@@ -542,6 +553,16 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1ced73b1dacfc750a6db6c0a0c3a3853c8b41997e2e2c563dc90804ae6867959"
 
+[[package]]
+name = "flate2"
+version = "1.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dc5a4e564e38c699f2880d3fda590bedc2e69f3f84cd48b457bd892ce61d0aa9"
+dependencies = [
+ "crc32fast",
+ "miniz_oxide",
+]
+
 [[package]]
 name = "fnv"
 version = "1.0.7"
@@ -1243,6 +1264,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1fa76a2c86f704bdb222d66965fb3d63269ce38518b83cb0575fca855ebb6316"
 dependencies = [
  "adler2",
+ "simd-adler32",
 ]
 
 [[package]]
@@ -1769,6 +1791,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "simd-adler32"
+version = "0.3.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d66dc143e6b11c1eddc06d5c423cfc97062865baf299914ab64caa38182078fe"
+
 [[package]]
 name = "similar"
 version = "2.7.0"