feat: S3-compatible object storage backend

BACKEND_STORAGE_DRIVER=s3 swaps LocalStore for a stdlib-only SigV4
implementation — PutObject/GetObject/DeleteObject/CopyObject/ListObjectsV2
against any S3-compatible API (AWS, MinIO, Garage, R2). Storage becomes a
Store interface; storage_path records s3://bucket/key. Content-Length is
resolved by seeking or temp-file spooling — chunked uploads are rejected
by real S3 servers. Bucket auto-created on startup.

Verified live against zenko/cloudserver: bucket, put, get, copy, delete.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -23,6 +23,7 @@ All notable changes to Primora. Format follows [Keep a Changelog](https://keepac
 - **Document filtering and ordering** — `GET /collections/:id/documents` accepts `filter` (comma-separated `field.op.value`; ops `eq neq gt gte lt lte like in is`; dotted paths index into document data, `id`/`created_at`/`updated_at` filterable) and `order` (`field.asc|field.desc` list). Whitelisted parsing + parameterized values; plain lists unchanged. CLI: `primora documents list --collection <slug> --filter … --order …`.
 - **HA scheduler leadership** — the job scheduler and alert evaluator each hold a session-scoped PostgreSQL advisory lock on a dedicated pinned connection, so multiple backend replicas scan for due work exactly once; manual and hook-triggered runs remain available on any replica. Leadership is released on shutdown and fails over automatically when a replica dies — verified live with a two-replica test.
 - **Email templates + send log** (`core.email_log`, migration 00014) — the mailer renders registered templates (currently `invitation`) and every send — successful or failed — is recorded with template, recipient, subject, status, and error text. `GET /projects/:id/emails` lists recent sends; the Members page shows an "Email log" card under pending invitations.
+- **S3-compatible object storage backend** — `BACKEND_STORAGE_DRIVER=s3` swaps the local disk store for any S3-compatible API (AWS, MinIO, Garage, R2) via a stdlib-only SigV4 implementation — no AWS SDK dependency. `S3_ENDPOINT`/`S3_REGION`/`S3_BUCKET`/credentials configure it; `S3_PATH_STYLE` (default on) covers MinIO/Garage, off for AWS virtual-hosted. The configured bucket is created on startup if missing; objects stream with computed SHA-256, `storage_path` records `s3://bucket/key`. Verified live end-to-end (bucket → put → get → copy → list → delete).
 
 ## [0.7.1] - 2026-09-23