refactor(storage): remove all PostgreSQL dependencies and code

Remove the entire PostgreSQL storage layer, keeping only P2P/local
storage backends (sled chain storage + SQLite local storage).

Deleted files:
- storage/src/pg.rs (connection pool: PgConfig, PgPool, create_pool)
- storage/src/postgres/ directory (mod.rs, evaluations.rs,
  leaderboard.rs, submissions.rs, task_logs.rs, validators.rs)

Updated files:
- storage/src/lib.rs: removed pub mod pg and pub mod postgres
- storage/src/traits.rs: removed From<tokio_postgres::Error> and
  From<deadpool_postgres::PoolError> impls
- storage/Cargo.toml: removed tokio-postgres, deadpool-postgres,
  postgres-types dependencies
- AGENTS.md: removed pg.rs and postgres/ from architecture section
- Cargo.lock: regenerated without PostgreSQL crates

Author: echobt

AGENTS.md

@@ -65,15 +65,7 @@ term-challenge/
 │       ├── lib.rs              # Root module, re-exports, From impls for StorageError
 │       ├── traits.rs           # ChallengeStorage trait, StorageError, Result alias
 │       ├── chain.rs            # Chain storage (sled)
-│       ├── local.rs            # Local storage (SQLite)
-│       ├── pg.rs               # PostgreSQL connection pool (deadpool-postgres)
-│       └── postgres/
-│           ├── mod.rs          # Submodule declarations
-│           ├── evaluations.rs  # Evaluation CRUD using EvaluationResult (f64 score, UUID ChallengeId)
-│           ├── leaderboard.rs  # Leaderboard queries using WeightAssignment (f64 weight)
-│           ├── submissions.rs  # Submission storage with ChallengeId (UUID) and Hotkey (SS58)
-│           ├── task_logs.rs    # Task log storage and retrieval
-│           └── validators.rs   # Validator management with Hotkey (SS58 encoding)
+│       └── local.rs            # Local storage (SQLite)
 ├── cli/
 │   ├── Cargo.toml      # native binary, ratatui TUI
 │   └── src/

Cargo.lock

@@ -11,9 +11,6 @@ platform-core = { workspace = true }
 platform-challenge-sdk = { workspace = true }
 
 tokio = { version = "1.40", features = ["full"] }
-tokio-postgres = { version = "0.7", features = ["with-uuid-1", "with-chrono-0_4", "with-serde_json-1"] }
-deadpool-postgres = { version = "0.14", features = ["serde"] }
-postgres-types = { version = "0.2", features = ["derive", "with-uuid-1", "with-chrono-0_4", "with-serde_json-1"] }
 
 sled = "0.34"
 rusqlite = { version = "0.31", features = ["bundled"] }
@@ -27,4 +24,4 @@ tracing = "0.1"
 hex = "0.4"
 
 [dev-dependencies]
-tempfile = "3"
+tempfile = "3"