feat(server): escrow preparation store (#96)

[moises-cisneros]

Refs #96

Part of the chain that replaces #119, split after its review. Tracker: #122.

Summary

Persistence of preparations (a new one supersedes the previous one) and the chain submission store additions the relay needs (transaction, listByHire). Each preparation has at most one ChainSubmission, proven by a Postgres race test.

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
dart test test/integration/escrow_preparation_store_test.dart test/integration/chain_submission_store_test.dart   (Postgres test container)

Notes for reviewers

• Review against the parent branch (feat/96-escrow-relay-04-rpc-client), not main.
• Over the 400 line budget: 936 authored lines, mostly tests of code that moves funds. It is one cohesive unit and cannot shrink without dropping tests, 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	5 of 12
Base	feat/96-escrow-relay-04-rpc-client
Depends on	#126
Follow-up	#128
Review budget	936 authored (936 total) / 400
Starts at	feat/96-escrow-relay-04-rpc-client
Ends with	The store interface, Serverpod and in-memory implementations, shared contract tests and the integration test.

Chain Overview

main
 └── #122 Tracker (draft, no merge)
      └── #123 protocol
           └── #124 xdr-encoder
                └── #125 stellar-codec
                     └── #126 rpc-client
                          └── 📍 #127 preparation-store
                               └── #128 hire-lifecycle
                                    └── #129 prepare
                                         └── #130 submit
                                              └── #131 hire-service
                                                   └── #132 endpoint
                                                        └── #133 docs
                                                             └── #134 openspec

Scope

• Includes: The store interface, Serverpod and in-memory implementations, shared contract tests and the integration test.
• Excludes: Hire lifecycle and services.

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

[moises-cisneros]

Folded into #129 to keep the chain at 4 PRs. The commits are the same, just grouped; this branch stays in the history of #129.

[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.