feat: Bepo v0.5 — POST /chat endpoint with shared search helper

[Copilot]

Adds a lightweight deterministic chat endpoint that answers natural-language questions about saved memories using local metadata — no LLM, no external APIs.

New: POST /chat

• JSON body: message (required, non-empty), top_k (1–10, default 3)
• Reuses memory scoring via extracted search_memory_matches(query, top_k) helper
• Returns status, answer, count, memories (safe fields only — no embeddings)
• Answer is built deterministically from top match: place_hint → bepo_summary/user_note/note → mood/tags → map hint

POST /chat
{"message": "Where was that calm cafe with the cat?", "top_k": 3}

{
  "status": "success",
  "answer": "You may mean the memory near the red couch hallway. I remember it as calm, cafe, cat, cozy.",
  "count": 1,
  "memories": [...]
}

Refactor: search_memory_matches helper

Extracted shared DB-query + cosine-scoring logic from /search into search_memory_matches(query, top_k) → list[dict]. Both /search and /chat delegate to it — no behavior change on /search.

Validation

• ChatRequest Pydantic model: @field_validator rejects whitespace-only messages; Field(ge=1, le=10) enforces top_k range — both return 422
• 9 new tests in TestChat covering empty DB, validation rejection, metadata in answer, embedding exclusion, and /search backward-compatibility

[Copilot]

Pull request overview

Adds a deterministic, local POST /chat endpoint that answers natural-language questions about saved memories by reusing search logic extracted into a shared helper. Bumps the app version to 0.5 and updates README.

Changes:

• Extract /search's scoring/DB-query logic into search_memory_matches(query, top_k), used by both /search and /chat with no behavior change on /search.
• Add ChatRequest Pydantic model (with whitespace and top_k 1–10 validation), build_chat_answer() helper, and the POST /chat endpoint that returns safe memory fields (no embeddings).
• Update root endpoint version to 0.5, register /chat, add README docs, and add a TestChat suite covering validation, no-results, metadata-in-answer, embedding exclusion, and /search backward compatibility.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
main.py	Adds ChatRequest, search_memory_matches, build_chat_answer, /chat endpoint; refactors /search to delegate; bumps version to 0.5.
tests/test_api.py	Adds TestChat covering empty DB, validation, metadata in answers, embedding non-exposure, and /search regression check.
README.md	Documents the new /chat endpoint with request/response examples.

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