fix(server): issues #115-#118 kick, empty text, SQLite busy

[Cod-e-Codes]

Summary

• Admin self-kick disconnects with no response and blocks reconnect for 24 hours #115: Reject self kick/ban; KickUser/BanUser return errors so callers claim success only on nil
• Kicking a username with no active connection reports success and creates a ban entry #116: Online-only KickUser (no offline temp ban); keep offline :ban
• Text messages with empty content are accepted, broadcast, and persisted #117: Reject empty/whitespace plaintext on text/dm/edit when not encrypted
• Concurrent message sends can fail to insert with SQLITE_BUSY and are not delivered #118: SQLite busy_timeout + WAL via modernc DSN; SetMaxOpenConns(1)

Fixes #115
Fixes #116
Fixes #117
Fixes #118

Test plan

Black-box against binaries built from this branch (temp server on localhost, raw WebSocket client, SQLite file DB):

• CI go.yml build + database-smoke green after each commit
• Self-kick rejected with System You cannot kick yourself.; connection stays open; reconnect works (no 24h ban)
• Self-ban rejected with System You cannot ban yourself.; reconnect works (not permanently banned)
• Offline kick returns user is not connected (not success); target can still connect (no ban)
• Online kick reports success, victim disconnects, reconnect gets close 1008 You are banned from this server
• Empty and whitespace-only plaintext rejected with Message not sent: empty content; Encrypted opaque content still accepted
• Concurrent load 20 clients x 10 messages: 200/200 writes, 200/200 rows persisted, 0 SQLITE_BUSY / insert-lock errors in server log

[Cod-e-Codes]

I re-checked #118 against the current implementation and primary docs.

Verdict: the current #118 fix is sufficient.

The fix applies SQLite pragmas through the modernc.org/sqlite DSN before opening the DB, including _busy_timeout, _journal_mode=WAL, _synchronous=NORMAL, and _pragma values. modernc.org/sqlite v1.55.0 documents those DSN keys and applies _busy_timeout early. The code then verifies busy_timeout, verifies WAL for file-backed DBs, skips WAL verification for memory DSNs, and limits SQLite to a single open/idle connection to serialize same-process writes.

The in-repo proof is TestInitDBSQLiteConcurrentInserts: it initializes SQLite through InitDB, creates the schema, runs 8 goroutines x 20 InsertMessage calls, asserts no insert errors, and checks COUNT(*) == 160.

The test proves the database-layer same-process concurrent insert path. It does not claim to be a full WebSocket fan-in reproduction or an external-process lock test.