feat(server): persist preparations and prepare unsigned escrow envelopes (#96)

[moises-cisneros]

Refs #96

Part of the chain that replaces #119, split after its review. Tracker: #122. Regrouped from a first 12 PR split (#127, #128 were folded into this one).

Summary

The stores and the prepare half of the relay. Preparation store (a new one supersedes the previous one; each preparation has at most one ChainSubmission, proven by a Postgres race test), the hire lifecycle store (idempotency key, input and job id; a hire has no status until create_job is confirmed), and prepareCreateJob, prepareFund, prepareComplete and prepareReject. The service has no submit logic yet.

Acceptance criteria

The chain as a whole covers #96; this PR carries its slice (see Scope below).

• The app can create and fund a hire on testnet using only prepare… and submitEscrowCall (proven in the docs PR)
• A signed envelope that differs from its preparation in any field is rejected
• A caller can never prepare or submit for a hire that is not theirs
• A submit… call never throws for chain outcomes
• Each preparation has at most one ChainSubmission

Verification evidence

dart analyze --fatal-infos
dart test test/unit test/hire
dart test test/integration/escrow_preparation_store_test.dart test/integration/chain_submission_store_test.dart test/integration/hire_lifecycle_store_test.dart   (Postgres test container)

Notes for reviewers

• Review against the parent branch (feat/96-escrow-relay-04-rpc-client), not main. The commits are ordered to be read one by one.
• Over the 400 line budget: 2976 authored lines, mostly tests of code that moves funds. Splitting further cuts the unit apart, so it needs size:exception.
• Generated code, migrations and recorded fixtures are excluded from the authored count.

Chain Context

Field	Value
Chain	Escrow relay prepare and submit endpoints (#96)
Tracker PR	#122
Position	2 of 4
Base	feat/96-escrow-relay-04-rpc-client
Depends on	#126
Follow-up	#132
Review budget	2976 authored (2976 total) / 400
Starts at	feat/96-escrow-relay-04-rpc-client
Ends with	Preparation and lifecycle stores with contract and integration tests, ChainAccounts, the prepare side of EscrowRelayService.

Chain Overview

main
 └── #122 Tracker (draft, no merge)
      └── #126 escrow relay foundations: protocol, config, envelope codec and RPC
           └── 📍 #129 persist preparations and prepare unsigned escrow envelopes
                └── #132 submit signed escrow calls and expose the relay in the hire endpoint
                     └── #134 escrow relay API, ADRs, README, testnet e2e and OpenSpec archive

Scope

• Includes: Preparation and lifecycle stores with contract and integration tests, ChainAccounts, the prepare side of EscrowRelayService.
• Excludes: submitEscrowCall, hire creation and the endpoint.

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests, docs, or manual verification cover this unit

[TOMOKI977]

Approved. This slice is part of the code I reviewed in full on #119 at b0a4adc (all four lenses; see the review there). I checked that the tip of the chain matches b0a4adc except for the CI fix in .env.example and .gitleaks.toml (and main's #112). The follow-ups listed on #119 apply to the chain as a whole.

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