Stellar base reserve and fee estimates: user-facing and API fields for pre-flight

[greatest0fallt1me]

Expose min_balance_xlm and estimated_fees on stream create preflight

    ## Description

    **Help users and wallets** avoid `op_underfunded` and **min reserve** surprises: add

pre-flight or GET that returns required XLM and per-tx fee estimates
for a proposed stream, including any Soroban and escrow overhead. This is
backend truth; the Figma side has a separate design issue in StreamPay-Frontend.

    ## Requirements and context

    - **Plain** `amounts` in stroops and human-readable; **rounding** policy repeated.

• Cite which Horizon/RPC calls back the numbers (and cache policy).

• Tests with fixture responses; no live network in unit tests.

• Rate limit: these can be high volume; add cache and limits.

• Security: no PII; only for authenticated user’s accounts or public estimate only if safe.

    ## Suggested execution
  
    1. `git checkout -b feature/preflight-reserve-estimates`
  

2. Add GET /streams/estimate or POST with dry-run.

3. PR with OpenAPI and example response for two assets.

4. Cross-link to contracts for exact lock amounts.

5. Timeframe: 96h; extend for multi-hop paths.

    ## Test and commit
   

• Run the full test suite; add or update tests until the agreed coverage bar is met.
• Cover edge cases listed in this issue; document any intentional exclusions with brief rationale in the PR.
• Include relevant test output (e.g. test runner summary) or a link to a passing CI run in the pull request.
• Add security notes for auth, keys, PII, chain settlement, or money movement (assumptions verified, out-of-scope items).

Example commit message

feat(stellar): add reserve and fee estimate preflight for stream and escrow planning

Guidelines

• Target: at least 95% coverage on new or meaningfully changed code (per the repo’s standard tooling).
• Documentation: update contributor-facing or API documentation where a reviewer would be blocked without it.
• Timeframe: 96 hours to ready-for-review (surface blockers early).

[2411015047-afk]

Hi! I can help implement the pre-flight API for XLM reserve and fee estimates.

I have experience with Stellar/Soroban backend development and Python scripting. This kind of fee estimation tool is straightforward to build — it involves:

• Querying Stellar RPC for recent fee stats
• Calculating minimum XLM reserve based on number of entries
• Returning clear estimates before stream creation

Let me know if you'd like to hire this out — I can deliver a clean, documented implementation.