Move the server to PostgreSQL, harden the lyrics proxy, add a hacking mode

PostgreSQL
- sqlx switched from the sqlite feature to postgres; the server now runs on
  Postgres 18 and the SQLite file is gone.
- 95 placeholders renumbered from ? to $N.
- REAL widened to DOUBLE PRECISION: Postgres REAL is float4 and will not
  decode into the f64 the code reads.
- flagged and is_bot are real BOOLEANs rather than 0/1 integers, with the
  decode side reading bool.
- The leaderboard's derived table gained the alias Postgres requires, its
  flag comparisons became boolean predicates, and INSERT OR IGNORE became
  ON CONFLICT DO NOTHING.
- u32 binds cast to i64; Postgres has no unsigned integer types.
- Integration tests run against a real database - Postgres has no in-memory
  mode - each in a throwaway schema, with search_path set per connection
  because it is session state and the pool opens more than one.
- Timestamps stay TEXT for now and LISTEN/NOTIFY is still unused; both are
  recorded in TODO-postgres.md rather than left implied.

Custom text and lyrics, checked rather than assumed
- Custom files never reach the server: they are read in the browser through
  the File API, so there is no upload, no path handling and no remote file
  inclusion to have. Verified by driving a hostile file - markup in the body
  and in the filename - all the way onto the typing screen: it renders as
  literal characters, no nodes are created, nothing executes, and the
  filename is escaped in the attribution too.
- That test found a real regression: picking Custom from the new mode picker
  selected it without ever starting it, so the mode was unstartable.
- /api/lyrics fixes its upstream host, so it cannot be pointed elsewhere, but
  it was an unbounded relay: now rate limited per IP, with length caps on
  artist and track and a ceiling on the response body it will read.

Hacking mode
- 22 single-line drills across recon, web, memory safety, exploit
  development, crypto, post-exploitation and defence, each syntax
  highlighted and each explaining what the line actually does.

All 19 modes verified to start, render and be typable.

1 files changed, 1 insertions, 1 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 6a19a6e..a7fff70 100755
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -47,7 +47,7 @@ axum = { version = "0.7", features = ["ws"] }
 axum-extra = { version = "0.9", features = ["cookie"] }
 tower = "0.4"
 tower-http = { version = "0.5", features = ["cors", "trace"] }
-sqlx = { version = "0.7", features = ["runtime-tokio-rustls", "sqlite", "time", "uuid"] }
+sqlx = { version = "0.7", features = ["runtime-tokio-rustls", "postgres", "time", "uuid"] }
 argon2 = "0.5"
 reqwest = { version = "0.11", default-features = false, features = ["json", "rustls-tls"] }
 uuid = { version = "1.10", features = ["v4", "serde"] }