fix(auth): enforce deterministic amount precision/overflow guarantees in recovery & session flows

[Neziahtech]

Closes #1642

Summary

Problem

<What was previously unguarded: e.g. no explicit scale/sign/overflow validation before
state mutation in <file/function>, allowing rounding or truncation to silently alter
stored/displayed amounts, or allowing stale/replayed auth state to reach protected paths.>

Design & Invariants

• Amount representation: <integer minor-units | fixed-point decimal | library used>
• Validated at the boundary: <function/endpoint>, before any state mutation
• Rules enforced: exact scale, non-negative/sign rules, min/max bounds, overflow rejection
• Rejection is atomic: invalid input never partially applies

Changes

• <path>:
• <path>:

Failure Behavior & Compatibility

• Invalid scale/sign/overflow → rejected with <error type/shape>, no state change
• Public API/response shape: <unchanged | changed as follows: ...>
• Migration/rollback: <none required | steps>

Test Evidence

Boundary values tested against independent oracle (<tool/lib>):

• zero
• minimum representable value
• maximum representable value
• near-overflow (max ± 1 unit)
• fractional / sub-minor-unit
• conversion boundary (e.g. currency/unit conversion rounding)

Concurrency/replay:

• repeated/duplicate request leaves no partial state
• stale/expired token rejected before mutation
• concurrent multi-tab/session case

Commands Run

```
<formatter command + result>
<lint command + result>
<type/build command + result>
<full test suite command + result>
<security/contract/migration check command + result, if applicable>
```

Security / Correctness Note

<1 short paragraph: what this guarantees, what it does not cover, any residual
operational limitation or assumption>

Scope Confirmation

• No unrelated refactors
• No formatting-only changes bundled in
• No generated artifacts or secrets included
• No disabled checks