feat: add post3 s3 proxy for postgresql

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/POST3-004-s3-server-skeleton.md

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+# POST3-004: S3 HTTP server skeleton
+
+**Status:** Done
+**Priority:** P0
+**Blocked by:** POST3-003
+
+## Description
+
+Build the post3-server binary with CLI (clap), state management, notmad component lifecycle, and axum router with all S3 routes wired up.
+
+## Acceptance Criteria
+
+- [ ] `main.rs` — dotenvy + tracing_subscriber + cli::execute()
+- [ ] `cli.rs` — clap App with `serve` subcommand
+- [ ] `cli/serve.rs` — ServeCommand with --host flag, starts notmad::Mad with S3Server
+- [ ] `state.rs` — State struct (PgPool + Store), runs migrations on new()
+- [ ] `s3/mod.rs` — S3Server implementing notmad::Component
+- [ ] `s3/router.rs` — all 9 routes mapped to handler functions
+- [ ] Server starts, binds to port, responds to requests
+- [ ] `cargo check -p post3-server` passes