Feat/user discovery

[nathanaelcheramlak]

Summary
Adds username discovery and server-backed direct messaging, replacing the TUI’s local-only “New DM” placeholder.
Users can now press n, search by username prefix, select a person, and open their existing conversation or create a new one.
Changes

• Adds authenticated GET /api/v1/users/search?q=... with case-insensitive prefix matching, query validation, rate limiting, and a 20-result limit.
• Returns only public fields (id, name, username) and excludes the requesting user.
• Treats SQL wildcard characters literally and ranks exact username matches first.
• Connects the TUI to the existing conversation-creation endpoint, using real conversation and recipient IDs.
• Adds debounced search, keyboard selection, loading/error/empty states, cancellation, and stale-response protection.
• Prevents duplicate submissions and reuses existing conversations without duplicating sidebar entries or overwriting live chat metadata.
• Fixes the existing SQL visibility/conflict-handling bug affecting first-time and concurrent DM creation.
• Documents the discovery workflow and database integration-test requirements.
  User Flow

1. Press Esc to leave typing mode, then n.
2. Enter at least two characters of a username.
3. Select a result with the up/down arrows.
4. Press Enter to open or create the conversation.
5. Press i to start typing, or Esc to cancel discovery.
   Testing

• Each code commit passed make test against an isolated staged snapshot.
• Final verification passed make fmt, make test GOFLAGS=-race, and make vet.
• Both binaries built successfully using an explicit verification-only shared secret.
• PostgreSQL integration tests passed during implementation, covering search ordering, literal wildcard matching, password-free column access, first-time creation, conversation reuse, and concurrent creation resolving to a single DM.
• Added HTTP client and TUI model tests for cancellation, stale sessions/replies, duplicate submission prevention, error handling, and responsive rendering.

[DanielJohn17]

Added migration 20261005212029_add_idx_users_lower_username_pattern.sql creating a functional index on users (LOWER(username) text_pattern_ops).

This enables PostgreSQL to perform fast (\log N)$ B-Tree index range scans for LOWER(username) LIKE LOWER(...) ESCAPE '\' prefix queries instead of running a full table scan on every search keystroke.