refactor: extract adapter-common crate — shared sqlx error mapping,

row-to-domain conversions, date/uuid utils

Eliminates 39 map_err copies, consolidates parse_uuid/parse_datetime/
datetime_to_str/format_year_month, extracts 7 shared row-to-domain
conversion functions (movie, review, watchlist, stats, user_summary).
Row structs stay per-adapter (FromRow is db-specific), only conversion
logic is shared. Net -281 lines.

crates/adapters/postgres/Cargo.toml

@@ -11,6 +11,7 @@ sqlx = { version = "0.8.6", features = [
     "macros",
     "chrono",
 ] }
+adapter-common = { workspace = true }
 domain      = { workspace = true }
 postgres-federation = { workspace = true }
 anyhow      = { workspace = true }