fix: contracts workspace cleanup and README merge marker

[privexlabs]

Summary

• Remove leftover git merge conflict marker from README.md
• Add the 7 missing contract crates to the Cargo workspace and fix the compile errors that surfaced (including
  two pre-existing broken members, did_registry and payment_gateway, that were blocking a clean cargo check
  across the workspace)
• Remove contracts/src/, ~76 orphaned single-file contract templates documented in contracts/PROGRESS.md as
  never part of any workspace crate, along with the stale errors.json/errors2.json/errors3.json logs
• Configure local .env files for the backend (Postgres + Redis, matching docker-compose's host port mappings) —
  not committed, .env is git-ignored

Details

• contracts/Cargo.toml: added continuous_bonding_curve, dao_governance, fractional_nft_vault, lending_pool,
  parametric_insurance, zk_proof_verifier, hello_world to [workspace] members, alphabetized the list, and
  dropped the orphaned root [package] (nothing in the repo depended on it)
• Removed each new member's redundant standalone [workspace] table so they join the outer workspace
• did_registry: modernized to current soroban-sdk (added #[contract], replaced removed env.invoker() with
  explicit require_auth() params, switched storage().get/set to storage().persistent())
• payment_gateway: fixed assert! misuse (error enum passed where a format string was expected), symbol_short!
  length limits, u32/u64 timestamp mismatches, a borrow-after-move
• zk_proof_verifier: fixed a Hash<32>/BytesN<32> comparison and an Address serialization call that no longer
  exists in this SDK version
• hello_world: added the missing alloc::string::ToString import
• cargo check --workspace, cargo build --workspace, and cargo fmt --check all pass clean

Verified

• cargo check --workspace — no errors
• cargo build --workspace — matches CI's build step
• cargo fmt --check — no diffs
• README no longer contains >>>>>>>
• Backend starts and passes env validation against dockerized Postgres/Redis with a locally configured .env
  (not committed)

Closes #944
Closes #943
Closes #942
Closes #940

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[ayomideadeniran]

Pr under review