feat: add post3 s3 proxy for postgresql

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

todos/POST3-009-ci-dagger.md

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+# POST3-009: CI pipeline with Dagger Rust SDK
+
+**Status:** Done
+**Priority:** P1
+**Blocked by:** —
+
+## Description
+
+Set up a Dagger-based CI pipeline using a custom self-contained `ci/` crate with `dagger-sdk` directly (not the external `cuddle-ci` / `dagger-rust` components, which are too opinionated for post3's context).
+
+## What was built
+
+- [x] `ci/` added as workspace member
+- [x] `ci/Cargo.toml` with dependencies: dagger-sdk, eyre, tokio, clap
+- [x] `ci/src/main.rs` — custom pipeline with:
+  - `pr` and `main` subcommands (clap CLI)
+  - Source loading with dependency caching (skeleton files pattern from dagger-components)
+  - `rustlang/rust:nightly` base with clang + mold 2.3.3 for fast linking
+  - Dagger cache volumes for target/ and cargo registry
+  - `cargo check --workspace` compilation check
+  - PostgreSQL 18 as Dagger service container for integration tests
+  - `cargo test --workspace -- --test-threads=1` against Dagger PG
+  - Release binary build + packaging into `debian:bookworm-slim`
+  - `post3-server --help` sanity check in final image
+- [x] `mise.toml` tasks: `ci:pr`, `ci:main`
+- [x] No container publish (deferred until registry is decided)
+
+## Reference
+
+Pattern inspired by dagger-components (`/home/kjuulh/git/git.kjuulh.io/kjuulh/dagger-components`) but self-contained — no external git dependencies.