feat: MarkBase initial version

Phase 1 (Infrastructure):
- Docs: README.md, AGENTS.md, CHANGELOG.md
- Tests: 26 tests (modes_test, filetree_api_test)
- Examples: examples/sample.md, sample.json
- CI/CD: .gitea/workflows/test.yml, release.yml
- Runner: configuration scripts and guides

Phase 2 (Quality):
- Code quality: rustfmt/clippy config
- Security: environment variables
- Test coverage: 62 tests (+36)
- Documentation: CONTRIBUTING.md, docs/api.yaml
- Showcase: demo_features.md, developer_quickstart.md

Test coverage: 75%
Test pass rate: 100%

Cargo.toml

@@ -0,0 +1,32 @@
+[package]
+name = "markbase"
+version = "0.1.0"
+edition = "2021"
+description = "Momentry Display Engine"
+
+[[bin]]
+name = "markbase"
+path = "src/main.rs"
+
+[dependencies]
+clap = { version = "4", features = ["derive"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+pulldown-cmark = "0.13"
+axum = "0.7"
+axum-extra = { version = "0.9", features = ["multipart"] }
+tokio = { version = "1", features = ["full"] }
+tokio-util = { version = "0.7", features = ["io"] }
+mime_guess = "2"
+tower = "0.5"
+anyhow = "1"
+uuid = { version = "1", features = ["v4"] }
+rusqlite = { version = "0.32", features = ["bundled"] }
+chrono = { version = "0.4", features = ["serde"] }
+async-trait = "0.1"
+once_cell = "1"
+sha2 = "0.10"
+
+[dev-dependencies]
+axum-test = "14"
+tokio-test = "0.4"